parser.c: optimize json_string_unescape

[samyron]

This PR optimizes json_string_unescape.

Two commits:

1. Use ARM Neon to scan for \. While scanning, copy the current chunk to the output.
2. Add a fast path when unescaping a single character.

If this PR is accepted, I will follow up with an SSE2 implementation.

Benchmarks

Run on a Macbook Air M1.

twitterescaped.json is from simdjson-data.

== Parsing activitypub.json (58160 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     1.103k i/100ms
Calculating -------------------------------------
               after     11.143k (± 0.8%) i/s   (89.74 μs/i) -     56.253k in   5.048516s

Comparison:
              before:    10366.8 i/s
               after:    11143.2 i/s - 1.07x  faster

== Parsing twitterescaped.json (562408 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    73.000 i/100ms
Calculating -------------------------------------
               after    737.341 (± 0.9%) i/s    (1.36 ms/i) -      3.723k in   5.049667s

Comparison:
              before:      712.1 i/s
               after:      737.3 i/s - 1.04x  faster

I should note that the fast path for unescaping a single character accounts for about 1% of the speed increase in activitypub.json. It's pretty minor.

[byroot]

Sorry for the delay, I just started a new work and was busy.

This PR is interesting, but while reviewing it, it gave me another idea: #902

We already find the \ during parsing, so we could actually record them to pass them to the decoder. Of course there is a space tradeoff, but that's the idea.

With a handcrafted benchmark:

benchmark_parsing "some_unescape", JSON.dump([((" "*100) + "\n")*15])
benchmark_parsing "more_unescape", JSON.dump([((" "*100) + "\n")*30])

My PR perform significantly better:

== Parsing some_unescape (1534 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   252.233k i/100ms
Calculating -------------------------------------
               after      2.619M (± 0.6%) i/s  (381.76 ns/i) -     13.116M in   5.007434s

Comparison:
              before:  3159184.8 i/s
               after:  2619427.0 i/s - 1.21x  slower


== Parsing more_unescape (3064 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   153.779k i/100ms
Calculating -------------------------------------
               after      1.579M (± 0.7%) i/s  (633.23 ns/i) -      7.997M in   5.063882s

Comparison:
              before:  1796951.2 i/s
               after:  1579212.5 i/s - 1.14x  slower

(after is your branch, before is #902).

So I think I'll go with #902