Elide JSON::Parser allocation

Similar to #662, but here
we don't even need to spill on the heap, because the parser is never
exposed.

Before:

```
== Parsing small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json   188.233k i/100ms
                  oj   213.985k i/100ms
           oj strict   242.564k i/100ms
          Oj::Parser   448.682k i/100ms
           rapidjson   291.925k i/100ms
Calculating -------------------------------------
                json      1.983M (± 0.5%) i/s  (504.32 ns/i) -      9.976M in   5.031352s
                  oj      2.334M (± 0.2%) i/s  (428.48 ns/i) -     11.769M in   5.042839s
           oj strict      2.689M (± 0.2%) i/s  (371.85 ns/i) -     13.584M in   5.051044s
          Oj::Parser      4.662M (± 1.2%) i/s  (214.50 ns/i) -     23.331M in   5.005414s
           rapidjson      3.110M (± 0.7%) i/s  (321.57 ns/i) -     15.764M in   5.069531s

Comparison:
                json:  1982878.1 i/s
          Oj::Parser:  4661924.8 i/s - 2.35x  faster
           rapidjson:  3109722.2 i/s - 1.57x  faster
           oj strict:  2689277.0 i/s - 1.36x  faster
                  oj:  2333852.9 i/s - 1.18x  faster
```

After:

```
== Parsing small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json   223.083k i/100ms
                  oj   214.400k i/100ms
           oj strict   243.519k i/100ms
          Oj::Parser   445.445k i/100ms
           rapidjson   293.936k i/100ms
Calculating -------------------------------------
                json      2.279M (± 4.5%) i/s  (438.71 ns/i) -     11.377M in   5.002132s
                  oj      2.315M (± 0.3%) i/s  (431.96 ns/i) -     11.578M in   5.001141s
           oj strict      2.665M (± 0.9%) i/s  (375.19 ns/i) -     13.394M in   5.025562s
          Oj::Parser      4.703M (± 0.3%) i/s  (212.63 ns/i) -     23.609M in   5.019913s
           rapidjson      3.129M (± 0.4%) i/s  (319.55 ns/i) -     15.873M in   5.072213s

Comparison:
                json:  2279385.2 i/s
          Oj::Parser:  4703032.3 i/s - 2.06x  faster
           rapidjson:  3129356.1 i/s - 1.37x  faster
           oj strict:  2665318.3 i/s - 1.17x  faster
                  oj:  2315009.3 i/s - same-ish: difference falls within error
```

Author: byroot