[ruby/json] Optimize `fbuffer_append_long`

Ref: https://github.com/ruby/json/issues/655

Rather than to write the number backward, and then reverse
the buffer, we can start from the back of the buffer and write
the number in the proper direction.

Before:

```
== Encoding integers (8009 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json     8.606k i/100ms
                  oj     9.598k i/100ms
Calculating -------------------------------------
                json     86.059k (± 0.8%) i/s   (11.62 μs/i) -    430.300k in   5.000416s
                  oj     97.409k (± 0.6%) i/s   (10.27 μs/i) -    489.498k in   5.025360s

Comparison:
                json:    86058.8 i/s
                  oj:    97408.8 i/s - 1.13x  faster
```

After:

```
== Encoding integers (8009 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)     9.500k i/100ms
                json     9.359k i/100ms
                  oj     9.722k i/100ms
Calculating -------------------------------------
        json (reuse)     96.270k (± 0.4%) i/s   (10.39 μs/i) -    484.500k in   5.032777s
                json     94.800k (± 2.2%) i/s   (10.55 μs/i) -    477.309k in   5.037495s
                  oj     97.131k (± 0.7%) i/s   (10.30 μs/i) -    486.100k in   5.004822s

Comparison:
        json (reuse):    96270.1 i/s
                  oj:    97130.5 i/s - same-ish: difference falls within error
                json:    94799.9 i/s - same-ish: difference falls within error
```

https://github.com/ruby/json/commit/0655b58d14

0 files changed, 0 insertions, 0 deletions