Improve performance of Oj.load with symbol_keys mode (#681)

Convert from String to Symbol, seems that it has some overhead.
This patch will generate Symbol via `rb_intern3()` and `ID2SYM()`.

The same approach seems to be favored by other gems.
(https://github.com/brianmario/mysql2/blob/ca883e1a359a10f48ac5d0ce649827b424110cd7/ext/mysql2/result.c#L171-L172)

−               | before   | after    | result
--               | --       | --       | --
Oj.load          | 339.194k | 387.595k | 1.14x

### Environment
- MacBook Air (M1, 2020)
- macOS 12.0 beta 3
- Apple M1
- Ruby 3.0.2

### Before
```
Warming up --------------------------------------
             Oj.load    33.731k i/100ms
Calculating -------------------------------------
             Oj.load    339.194k (± 0.9%) i/s -      1.720M in   5.072086s
```

### After
```
Warming up --------------------------------------
             Oj.load    39.129k i/100ms
Calculating -------------------------------------
             Oj.load    387.595k (± 1.0%) i/s -      1.956M in   5.048159s
```

### Test code
```ruby
require 'benchmark/ips'
require 'oj'

json =<<-EOF
{
  "$id": "https://example.com/person.schema.json",
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Person",
  "type": "object",
  "properties": {
    "firstName": {
      "type": "string",
      "description": "The person's first name."
    },
    "lastName": {
      "type": "string",
      "description": "The person's last name."
    },
    "age": {
      "description": "Age in years which must be equal to or greater than zero.",
      "type": "integer",
      "minimum": 0
    }
  }
}
EOF

Benchmark.ips do |x|
  x.report('Oj.load') { Oj.load(json, symbol_keys: true) }
end
```

ext/oj/compat.c

@@ -27,19 +27,18 @@ static void hash_set_cstr(ParseInfo pi, Val kval, const char *str, size_t len, c
 
         if (Qundef == rkey) {
             if (Yes != pi->options.cache_keys) {
-                rkey = rb_str_new(key, klen);
-                rkey = oj_encode(rkey);
                 if (Yes == pi->options.sym_key) {
-                    rkey = rb_str_intern(rkey);
+                    rkey = ID2SYM(rb_intern3(key, klen, oj_utf8_encoding));
+                } else {
+                    rkey = rb_str_new(key, klen);
+                    rkey = oj_encode(rkey);
                 }
             } else {
                 VALUE *slot;
 
                 if (Yes == pi->options.sym_key) {
                     if (Qnil == (rkey = oj_sym_hash_get(key, klen, &slot))) {
-                        rkey  = rb_str_new(key, klen);
-                        rkey  = oj_encode(rkey);
-                        rkey  = rb_str_intern(rkey);
+                        rkey  = ID2SYM(rb_intern3(key, klen, oj_utf8_encoding));
                         *slot = rkey;
                         rb_gc_register_address(slot);
                     }

ext/oj/custom.c

@@ -959,12 +959,13 @@ static void hash_set_cstr(ParseInfo pi, Val kval, const char *str, size_t len, c
         volatile VALUE rstr = rb_str_new(str, len);
 
         if (Qundef == rkey) {
-            rkey = rb_str_new(key, klen);
-            rstr = oj_encode(rstr);
-            rkey = oj_encode(rkey);
             if (Yes == pi->options.sym_key) {
-                rkey = rb_str_intern(rkey);
+                rkey = ID2SYM(rb_intern3(key, klen, oj_utf8_encoding));
+            } else {
+                rkey = rb_str_new(key, klen);
+                rkey = oj_encode(rkey);
             }
+            rstr = oj_encode(rstr);
         }
         if (Yes == pi->options.create_ok && NULL != pi->options.str_rx.head) {
             VALUE clas = oj_rxclass_match(&pi->options.str_rx, str, (int)len);

ext/oj/object.c

@@ -59,14 +59,13 @@ static VALUE calc_hash_key(ParseInfo pi, Val kval, char k1) {
     }
 #else
     if (':' == k1) {
-        rkey = rb_str_new(kval->key + 1, kval->klen - 1);
-        rkey = oj_encode(rkey);
-	rkey  = rb_str_intern(rkey);
+        rkey = ID2SYM(rb_intern3(kval->key + 1, kval->klen - 1, oj_utf8_encoding));
     } else {
-        rkey = rb_str_new(kval->key, kval->klen);
-        rkey = oj_encode(rkey);
         if (Yes == pi->options.sym_key) {
-            rkey = rb_str_intern(rkey);
+            rkey = ID2SYM(rb_intern3(kval->key, kval->klen, oj_utf8_encoding));
+        } else {
+            rkey = rb_str_new(kval->key, kval->klen);
+            rkey = oj_encode(rkey);
         }
     }
 #endif
@@ -78,9 +77,7 @@ static VALUE str_to_value(ParseInfo pi, const char *str, size_t len, const char
     volatile VALUE rstr = Qnil;
 
     if (':' == *orig && 0 < len) {
-        rstr = rb_str_new(str + 1, len - 1);
-        rstr = oj_encode(rstr);
-        rstr = rb_funcall(rstr, oj_to_sym_id, 0);
+        rstr = ID2SYM(rb_intern3(str + 1, len - 1, oj_utf8_encoding));
     } else if (pi->circ_array && 3 <= len && '^' == *orig && 'r' == orig[1]) {
         long i = read_long(str + 2, len - 2);
 
@@ -259,9 +256,7 @@ static int hat_cstr(ParseInfo pi, Val parent, Val kval, const char *str, size_t
             parent->odd_args = oj_odd_alloc_args(odd);
         } break;
         case 'm':
-            parent->val = rb_str_new(str + 1, len - 1);
-            parent->val = oj_encode(parent->val);
-            parent->val = rb_funcall(parent->val, oj_to_sym_id, 0);
+            parent->val = ID2SYM(rb_intern3(str + 1, len - 1, oj_utf8_encoding));
             break;
         case 's':
             parent->val = rb_str_new(str, len);

ext/oj/strict.c

@@ -39,10 +39,11 @@ VALUE oj_calc_hash_key(ParseInfo pi, Val parent) {
         return rkey;
     }
     if (Yes != pi->options.cache_keys) {
-        rkey = rb_str_new(parent->key, parent->klen);
-        rkey = oj_encode(rkey);
         if (Yes == pi->options.sym_key) {
-            rkey = rb_str_intern(rkey);
+            rkey = ID2SYM(rb_intern3(parent->key, parent->klen, oj_utf8_encoding));
+        } else {
+            rkey = rb_str_new(parent->key, parent->klen);
+            rkey = oj_encode(rkey);
         }
         OBJ_FREEZE(rkey);
         return rkey;
@@ -51,9 +52,7 @@ VALUE oj_calc_hash_key(ParseInfo pi, Val parent) {
 
     if (Yes == pi->options.sym_key) {
         if (Qnil == (rkey = oj_sym_hash_get(parent->key, parent->klen, &slot))) {
-            rkey  = rb_str_new(parent->key, parent->klen);
-            rkey  = oj_encode(rkey);
-            rkey  = rb_str_intern(rkey);
+            rkey  = ID2SYM(rb_intern3(parent->key, parent->klen, oj_utf8_encoding));
             *slot = rkey;
             rb_gc_register_address(slot);
         }