Stop BigDecimal-specific optimization.  Instead, it tries the conversion
methods in the following order:

1. `try_convert`,
2. `new`, and
3. class-named function, e.g. `Foo::Bar.Baz` function for `Foo::Bar::Baz` class

If all the above candidates are unavailable, it fallbacks to Float.

If set to true all parsed objects will be
immediately frozen, and strings will be
deduplicated if the Ruby implementation
allows it.

When use non-frozen string for hash key with `rb_hash_aset()`, it will duplicate and freeze it internally.
To avoid duplicate and freeze, this patch will give a frozen string in `rb_hash_aset()`.

```
Warming up --------------------------------------
                json    14.000  i/100ms
Calculating -------------------------------------
                json    148.844  (± 1.3%) i/s -    756.000  in   5.079969s
```

```
Warming up --------------------------------------
                json    16.000  i/100ms
Calculating -------------------------------------
                json    165.608  (± 1.8%) i/s -    832.000  in   5.025367s
```

```
require 'json'
require 'securerandom'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id": i,
    "uuid": SecureRandom.uuid,
    "created_at": Time.now
  }
end

json = obj.to_json

Benchmark.ips do |x|
  x.report "json" do |iter|
    count = 0
    while count < iter
      JSON.parse(json)
      count += 1
    end
  end
end
```

https://github.com/flori/json/commit/18292c0c1d

Ragel generates a code `0 <= (*p)` where `*p` is char.
As char is unsigned by default on arm and RISC-V, it is warned by gcc:

```
compiling parser.c
parser.c: In function ‘JSON_parse_string’:
parser.c:1566:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  if ( 0 <= (*p) && (*p) <= 31 )
  ^
parser.c:1596:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  if ( 0 <= (*p) && (*p) <= 31 )
  ^
```

This change removes the warning by substituting the condition with
`0 <= (signed char)(*p)`.

To fix build failures.

This shall fix compile errors.