Only use the key cache if the Hash is in an Array

[casperisfine]

Otherwise the likeliness of seeing that key again is really low, and looking up the cache is just a waste.

Before:

== Parsing small hash (65 bytes)
ruby 3.4.0dev (2024-11-13T12:32:57Z fstr-update-callba.. 9b44b455b3) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
                json   343.049k i/100ms
                  oj   213.943k i/100ms
          Oj::Parser    31.583k i/100ms
           rapidjson   303.433k i/100ms
Calculating -------------------------------------
                json      3.704M (± 1.5%) i/s  (270.01 ns/i) -     18.525M in   5.003078s
                  oj      2.200M (± 1.1%) i/s  (454.46 ns/i) -     11.125M in   5.056526s
          Oj::Parser    285.369k (± 4.8%) i/s    (3.50 μs/i) -      1.453M in   5.103866s
           rapidjson      3.216M (± 1.6%) i/s  (310.95 ns/i) -     16.082M in   5.001973s

Comparison:
                json:  3703517.4 i/s
           rapidjson:  3215983.0 i/s - 1.15x  slower
                  oj:  2200417.1 i/s - 1.68x  slower
          Oj::Parser:   285369.1 i/s - 12.98x  slower

== Parsing test from oj (258 bytes)
ruby 3.4.0dev (2024-11-13T12:32:57Z fstr-update-callba.. 9b44b455b3) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
                json    54.539k i/100ms
                  oj    41.473k i/100ms
          Oj::Parser    24.064k i/100ms
           rapidjson    51.466k i/100ms
Calculating -------------------------------------
                json    549.386k (± 1.6%) i/s    (1.82 μs/i) -      2.781M in   5.064316s
                  oj    417.003k (± 1.3%) i/s    (2.40 μs/i) -      2.115M in   5.073047s
          Oj::Parser    226.500k (± 4.7%) i/s    (4.42 μs/i) -      1.131M in   5.005466s
           rapidjson    526.124k (± 1.0%) i/s    (1.90 μs/i) -      2.676M in   5.087176s

Comparison:
                json:   549385.6 i/s
           rapidjson:   526124.3 i/s - 1.04x  slower
                  oj:   417003.4 i/s - 1.32x  slower
          Oj::Parser:   226500.4 i/s - 2.43x  slower

After:

== Parsing small hash (65 bytes)
ruby 3.4.0dev (2024-11-13T12:32:57Z fstr-update-callba.. 9b44b455b3) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
                json   361.394k i/100ms
                  oj   217.203k i/100ms
          Oj::Parser    28.855k i/100ms
           rapidjson   303.404k i/100ms
Calculating -------------------------------------
                json      3.859M (± 2.9%) i/s  (259.13 ns/i) -     19.515M in   5.061302s
                  oj      2.191M (± 1.6%) i/s  (456.49 ns/i) -     11.077M in   5.058043s
          Oj::Parser    315.132k (± 7.1%) i/s    (3.17 μs/i) -      1.587M in   5.065707s
           rapidjson      3.156M (± 4.0%) i/s  (316.88 ns/i) -     15.777M in   5.008949s

Comparison:
                json:  3859046.5 i/s
           rapidjson:  3155778.5 i/s - 1.22x  slower
                  oj:  2190616.0 i/s - 1.76x  slower
          Oj::Parser:   315132.4 i/s - 12.25x  slower

== Parsing test from oj (258 bytes)
ruby 3.4.0dev (2024-11-13T12:32:57Z fstr-update-callba.. 9b44b455b3) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
                json    55.682k i/100ms
                  oj    40.343k i/100ms
          Oj::Parser    25.119k i/100ms
           rapidjson    51.500k i/100ms
Calculating -------------------------------------
                json    555.808k (± 1.4%) i/s    (1.80 μs/i) -      2.784M in   5.010092s
                  oj    412.283k (± 1.7%) i/s    (2.43 μs/i) -      2.098M in   5.089900s
          Oj::Parser    279.306k (±13.3%) i/s    (3.58 μs/i) -      1.356M in   5.022079s
           rapidjson    517.177k (± 2.7%) i/s    (1.93 μs/i) -      2.626M in   5.082352s

Comparison:
                json:   555808.3 i/s
           rapidjson:   517177.1 i/s - 1.07x  slower
                  oj:   412283.2 i/s - 1.35x  slower
          Oj::Parser:   279306.5 i/s - 1.99x  slower