[Swift] Uses a pointer based stack instead of native array

[mustiikhalil]

Uses a pointer based stack instead of native array, the performance sometimes struggles when using native arrays thus switching to a custom structure gave the flexbuffer implementation a bit of a speed up. As shown below, twitter encoding became a bit faster, but we still struggle with the bigger chunks like Canada data set.

Stack:

│ Benchmarks:canada-encode-manual-FlexbufferSharedKeys (ns) *            │      3121 │      3254 │      3269 │      3285 │      3310 │      3418 │      3850 │       919 │
├────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Benchmarks:canada-encode-manual-FlexbufferSharedKeysAndStrings (ns) *  │      3036 │      3201 │      3234 │      3277 │      3293 │      3377 │      3782 │       928 │
├────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Benchmarks:twitter-encode-manual-FlexbufferSharedKeys (ns) *           │       265 │       296 │       297 │       299 │       302 │       313 │       365 │     10065 │
├────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Benchmarks:twitter-encode-manual-FlexbufferSharedKeysAndStrings (ns) * │       587 │       650 │       656 │       661 │       668 │       687 │       760 │      4569

Previous implementation:

│ Benchmarks:canada-encode-manual-FlexbufferSharedKeys (ns) *            │      3023 │      3238 │      3275 │      3344 │      3432 │      3518 │      3834 │       908 │
├────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Benchmarks:canada-encode-manual-FlexbufferSharedKeysAndStrings (ns) *  │      3230 │      3463 │      3521 │      3547 │      3588 │      3641 │      3759 │       855 │
├────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Benchmarks:twitter-encode-manual-FlexbufferSharedKeys (ns) *           │       300 │       330 │       336 │       340 │       345 │       353 │       423 │      8912 │
├────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Benchmarks:twitter-encode-manual-FlexbufferSharedKeysAndStrings (ns) * │       621 │       679 │       685 │       689 │       691 │       707 │       779 │      4388 │
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[aardappel]

Kinda sad their builtin arrays are implemented so badly that doing it custom is that much faster..

[mustiikhalil]

@aardappel they arent completely bad, but the issue is the hand holding they have in terms of their implementations. Thus they gave developers the option to use the RandomAcessCollection, which just an array but you have a bit of power in terms of how to use them