Use rb_sym2str() to retrieve String object (#687)

When it retrieves String object using `rb_sym_to_s()`, it duplicate the internal String of Symbol.

```c
VALUE
rb_sym_to_s(VALUE sym)
{
    return str_new_shared(rb_cString, rb_sym2str(sym));
}
```
Refer: https://github.com/ruby/ruby/blob/6f6a84f2f3e82f9554618f080f9b66ded52884cc/string.c#L11136-L11140

There will be no need to duplicate it, since we haven't changed the retrieved String.
This patch will improve performance where it handles Symbols.

−               | before   | after    | result
--               | --       | --       | --
Oj.dump          | 807.090k | 1.173M   | 1.45x
Oj.dump (compat) | 760.372k | 1.080M   | 1.42x
Oj.dump (rails)  | 656.976k | 880.951k | 1.34x

### Environment
- MacBook Air (M1, 2020)
- macOS 12.0 beta 4
- Apple M1
- Ruby 3.0.2

### Before
```
Warming up --------------------------------------
             Oj.dump    80.910k i/100ms
    Oj.dump (compat)    75.874k i/100ms
     Oj.dump (rails)    65.977k i/100ms
Calculating -------------------------------------
             Oj.dump    807.090k (± 0.9%) i/s -      4.046M in   5.012848s
    Oj.dump (compat)    760.372k (± 0.8%) i/s -      3.870M in   5.089417s
     Oj.dump (rails)    656.976k (± 0.8%) i/s -      3.299M in   5.021613s
```

### After
```
Warming up --------------------------------------
             Oj.dump   119.895k i/100ms
    Oj.dump (compat)   107.509k i/100ms
     Oj.dump (rails)    87.631k i/100ms
Calculating -------------------------------------
             Oj.dump      1.173M (± 0.5%) i/s -      5.875M in   5.007040s
    Oj.dump (compat)      1.080M (± 0.7%) i/s -      5.483M in   5.075621s
     Oj.dump (rails)    880.951k (± 0.9%) i/s -      4.469M in   5.073530s
```

### 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|
  data = Oj.load(json, symbol_keys: true)

  x.report('Oj.dump') { Oj.dump(data) }
  x.report('Oj.dump (compat)') { Oj.dump(data, mode: :compat) }
  x.report('Oj.dump (rails)') { Oj.dump(data, mode: :rails) }
end
```

ext/oj/custom.c

@@ -833,7 +833,7 @@ static void dump_struct(VALUE obj, int depth, Out out, bool as_ok) {
             v = rb_struct_aref(obj, INT2FIX(i));
 #endif
             if (ma != Qnil) {
-                volatile VALUE s = rb_sym_to_s(rb_ary_entry(ma, i));
+                volatile VALUE s = rb_sym2str(rb_ary_entry(ma, i));
 
                 name = RSTRING_PTR(s);
                 len  = (int)RSTRING_LEN(s);

ext/oj/dump.c

@@ -717,10 +717,7 @@ void oj_dump_str(VALUE obj, int depth, Out out, bool as_ok) {
 }
 
 void oj_dump_sym(VALUE obj, int depth, Out out, bool as_ok) {
-    // This causes a memory leak in 2.5.1. Maybe in other versions as well.
-    // const char	*sym = rb_id2name(SYM2ID(obj));
-
-    volatile VALUE s = rb_sym_to_s(obj);
+    volatile VALUE s = rb_sym2str(obj);
 
     oj_dump_cstr(RSTRING_PTR(s), (int)RSTRING_LEN(s), 0, 0, out);
 }

ext/oj/dump_object.c

@@ -208,7 +208,7 @@ static void dump_str(VALUE obj, int depth, Out out, bool as_ok) {
 }
 
 static void dump_sym(VALUE obj, int depth, Out out, bool as_ok) {
-    volatile VALUE s = rb_sym_to_s(obj);
+    volatile VALUE s = rb_sym2str(obj);
 
     oj_dump_cstr(RSTRING_PTR(s), (int)RSTRING_LEN(s), 1, 0, out);
 }
@@ -694,7 +694,7 @@ static void dump_struct(VALUE obj, int depth, Out out, bool as_ok) {
 
         *out->cur++ = '[';
         for (i = 0; i < cnt; i++) {
-            volatile VALUE s = rb_sym_to_s(rb_ary_entry(ma, i));
+            volatile VALUE s = rb_sym2str(rb_ary_entry(ma, i));
 
             name = RSTRING_PTR(s);
             len  = (int)RSTRING_LEN(s);

ext/oj/parser.c

@@ -1217,7 +1217,7 @@ static VALUE parser_new(VALUE self, VALUE mode) {
 
         switch (rb_type(mode)) {
         case RUBY_T_SYMBOL:
-            mode = rb_sym_to_s(mode);
+            mode = rb_sym2str(mode);
             // fall through
         case RUBY_T_STRING: ms = RSTRING_PTR(mode); break;
         default:
@@ -1290,7 +1290,7 @@ static VALUE parser_missing(int argc, VALUE *argv, VALUE self) {
 #endif
     switch (rb_type(rkey)) {
     case RUBY_T_SYMBOL:
-        rkey = rb_sym_to_s(rkey);
+        rkey = rb_sym2str(rkey);
         // fall through
     case RUBY_T_STRING: key = rb_string_value_ptr(&rkey); break;
     default: rb_raise(rb_eArgError, "option method must be a symbol or string");

ext/oj/rails.c

@@ -157,7 +157,7 @@ static void dump_struct(VALUE obj, int depth, Out out, bool as_ok) {
     assure_size(out, 2);
     *out->cur++ = '{';
     for (i = 0; i < cnt; i++) {
-        volatile VALUE s = rb_sym_to_s(rb_ary_entry(ma, i));
+        volatile VALUE s = rb_sym2str(rb_ary_entry(ma, i));
 
         name = RSTRING_PTR(s);
         len  = (int)RSTRING_LEN(s);

ext/oj/usual.c

@@ -903,7 +903,7 @@ static VALUE opt_decimal_set(ojParser p, VALUE value) {
     switch (rb_type(value)) {
     case T_STRING: mode = RSTRING_PTR(value); break;
     case T_SYMBOL:
-        s    = rb_sym_to_s(value);
+        s    = rb_sym2str(value);
         mode = RSTRING_PTR(s);
         break;
     default:
@@ -1020,7 +1020,7 @@ static VALUE opt_missing_class_set(ojParser p, VALUE value) {
     switch (rb_type(value)) {
     case T_STRING: mode = RSTRING_PTR(value); break;
     case T_SYMBOL:
-        s    = rb_sym_to_s(value);
+        s    = rb_sym2str(value);
         mode = RSTRING_PTR(s);
         break;
     default: