Use rb_enc_get() to retrieve encoding type

To reduce overhead, this patch use `rb_enc_get()` instead.

−               | before   | after    | result
--               | --       | --       | --
Oj.dump          | 1.319M   | 1.336M   | -
Oj.dump (compat) | 948.393k | 1.125M   | 1.19x
Oj.dump (rails)  | 815.509k | 943.996k | 1.16x

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

### Before
```
Warming up --------------------------------------
             Oj.dump   131.400k i/100ms
    Oj.dump (compat)    94.239k i/100ms
     Oj.dump (rails)    79.788k i/100ms
Calculating -------------------------------------
             Oj.dump      1.319M (± 0.7%) i/s -      6.701M in   5.080504s
    Oj.dump (compat)    948.393k (± 0.9%) i/s -      4.806M in   5.068098s
     Oj.dump (rails)    815.509k (± 0.8%) i/s -      4.149M in   5.087955s
```

### After
```
Warming up --------------------------------------
             Oj.dump   133.596k i/100ms
    Oj.dump (compat)   113.563k i/100ms
     Oj.dump (rails)    95.233k i/100ms
Calculating -------------------------------------
             Oj.dump      1.336M (± 1.0%) i/s -      6.680M in   5.000748s
    Oj.dump (compat)      1.125M (± 0.9%) i/s -      5.678M in   5.046279s
     Oj.dump (rails)    943.996k (± 0.8%) i/s -      4.762M in   5.044464s
```

### 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)
  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/dump.c

@@ -708,10 +708,10 @@ void oj_write_obj_to_stream(VALUE obj, VALUE stream, Options copts) {
 }
 
 void oj_dump_str(VALUE obj, int depth, Out out, bool as_ok) {
-    rb_encoding *enc = rb_to_encoding(rb_obj_encoding(obj));
+    rb_encoding *enc = rb_enc_get(obj);
 
-    if (rb_utf8_encoding() != enc) {
-        obj = rb_str_conv_enc(obj, enc, rb_utf8_encoding());
+    if (oj_utf8_encoding != enc) {
+        obj = rb_str_conv_enc(obj, enc, oj_utf8_encoding);
     }
     oj_dump_cstr(RSTRING_PTR(obj), (int)RSTRING_LEN(obj), 0, 0, out);
 }