Implement a fast path for integer parsing

[casperisfine]

rb_cstr2inum isn't very fast because it handles tons of different scenarios, and also require a NULL terminated string which forces us to copy the number into a secondary buffer.

But since the parser already computed the length, we can much more cheaply do this with a very simple function as long as the number is small enough to fit into a native type (long long).

If the number is too long, we can fallback to the rb_cstr2inum slowpath.

Before:

== Parsing citm_catalog.json (1727030 bytes)
ruby 3.4.0dev (2024-11-06T07:59:09Z precompute-hash-wh.. 7943f98a8a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
                json    40.000 i/100ms
                  oj    35.000 i/100ms
          Oj::Parser    45.000 i/100ms
           rapidjson    38.000 i/100ms
Calculating -------------------------------------
                json    425.941 (± 1.9%) i/s    (2.35 ms/i) -      2.160k in   5.072833s
                  oj    349.617 (± 1.7%) i/s    (2.86 ms/i) -      1.750k in   5.006953s
          Oj::Parser    464.767 (± 1.7%) i/s    (2.15 ms/i) -      2.340k in   5.036381s
           rapidjson    382.413 (± 2.4%) i/s    (2.61 ms/i) -      1.938k in   5.070757s

Comparison:
                json:      425.9 i/s
          Oj::Parser:      464.8 i/s - 1.09x  faster
           rapidjson:      382.4 i/s - 1.11x  slower
                  oj:      349.6 i/s - 1.22x  slower

After:

== Parsing citm_catalog.json (1727030 bytes)
ruby 3.4.0dev (2024-11-06T07:59:09Z precompute-hash-wh.. 7943f98a8a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
                json    46.000 i/100ms
                  oj    33.000 i/100ms
          Oj::Parser    45.000 i/100ms
           rapidjson    39.000 i/100ms
Calculating -------------------------------------
                json    462.332 (± 3.2%) i/s    (2.16 ms/i) -      2.346k in   5.080504s
                  oj    351.140 (± 1.1%) i/s    (2.85 ms/i) -      1.782k in   5.075616s
          Oj::Parser    473.500 (± 1.3%) i/s    (2.11 ms/i) -      2.385k in   5.037695s
           rapidjson    395.052 (± 3.5%) i/s    (2.53 ms/i) -      1.989k in   5.042275s

Comparison:
                json:      462.3 i/s
          Oj::Parser:      473.5 i/s - same-ish: difference falls within error
           rapidjson:      395.1 i/s - 1.17x  slower
                  oj:      351.1 i/s - 1.32x  slower

[casperisfine]

This isn't even a fast path to be honest, it's a very naive integer parsing function. Dunno if it's worth parsing multiple bytes at a time with __builtin_bswap*?

[casperisfine]

According to a profile, in citm_catalog which is the benchmark with the most integers, we're spending 0.6% in fast_parse_integer, and 1.8% in the overall JSON_parse_integer.

So maybe not worth it.