Commits on Aug 5, 2021

1. Improve Oj.load performance …

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

   

   Watson1978 committed Aug 5, 2021
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