Improve Oj.load performance

[Watson1978]

When use non-frozen string as hash key with rb_hash_aset(), it will duplicate and freeze the string internally.

static int
hash_aset_str(st_data_t *key, st_data_t *val, struct update_arg *arg, int existing)
{
    if (!existing && !RB_OBJ_FROZEN(*key)) {
	*key = rb_hash_key_str(*key);
    }
    return hash_aset(key, val, arg, existing);
}

Refer: https://github.com/ruby/ruby/blob/bda56a03a625793cb3fd110458c3f7323d73705e/hash.c#L2890-L2897

To avoid duplicate and freeze, this patch will give a frozen string in rb_hash_aset().

FYI)
If you use string as hash key, hash object always might have frozen string as key.

irb(main):001:0> hash = { "foo" => 42, bar: 55 }
=> {"foo"=>42, :bar=>55}
irb(main):002:0> hash.keys[0].frozen?
=> true
irb(main):003:0> hash.keys[1].frozen?
=> true

This patch has same approch with flori/json#345

−	before	after	result
Oj.load	335.122k	422.081k	1.26x

Environment

• MacBook Air (M1, 2020)
• macOS 12.0 beta 3
• Apple M1
• Ruby 3.0.2

Before

Warming up --------------------------------------
             Oj.load    33.829k i/100ms
Calculating -------------------------------------
             Oj.load    335.122k (± 0.9%) i/s -      1.691M in   5.047682s

After

Warming up --------------------------------------
             Oj.load    42.573k i/100ms
Calculating -------------------------------------
             Oj.load    422.081k (± 0.5%) i/s -      2.129M in   5.043373s

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) }
end

[ohler55]

Good to know. I'll take advantage of that knowledge to update other places in the new parser as well.