Improve performance of Oj.load with symbol_keys mode

[Watson1978]

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

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

[ohler55]

Very nice, thank you.

[Watson1978]

Thank you for your quick response!