Use RSTRING_PTR() to retrieve C string pointer

[Watson1978]

rb_string_value_ptr() attempts implicit type conversion with #to_str method.

char *
rb_string_value_ptr(volatile VALUE *ptr)
{
    VALUE str = rb_string_value(ptr);
    return RSTRING_PTR(str);
}

Refer: https://github.com/ruby/ruby/blob/f81964568f954495ad9a517066bd241f5db22059/string.c#L2319-L2324

However, we have already convert to string using #to_s method or check the type.
Therefore, This patch will call RSTRING_PTR() directly to reduce overhead.

−	before	after	result
Oj.load	419.273k	421.858k	-
Oj.dump	1.233M	1.303M	1.06x

Environment

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

Before

Warming up --------------------------------------
             Oj.load    42.892k i/100ms
             Oj.dump   124.603k i/100ms
Calculating -------------------------------------
             Oj.load    419.273k (± 0.6%) i/s -      2.102M in   5.012916s
             Oj.dump      1.233M (± 0.5%) i/s -      6.230M in   5.051849s

After

Warming up --------------------------------------
             Oj.load    42.986k i/100ms
             Oj.dump   132.017k i/100ms
Calculating -------------------------------------
             Oj.load    421.858k (± 0.4%) i/s -      2.149M in   5.094940s
             Oj.dump      1.303M (± 0.8%) i/s -      6.601M in   5.068088s

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) }

  data = Oj.load(json)
  x.report('Oj.dump') { Oj.dump(data) }
end