Compile-time evaluate constant BitArray int segments on JavaScript

[richard-viney]

Implements #3724.

Details

1. Ints in bit array expressions with a known value and size at compile time are now evaluated to bytes instead of using sizedInt() at runtime.

2. Ints in bit array patterns with a known value and size at compile time are now evaluated then matched byte-wise instead of using intFromSlice() at runtime.

3. The above is currently limited to Int sizes <= 48 bits in order to be under the JS safe integer limit, because otherwise:

   1. Code such as <<0:8192>> would evaluate to [0,0,0,0, ...]to 1024 entries, which seems undesirable (though I'm not sure if such code is commonly encountered).
   2. Code such as let assert <<0xFFFFFFFFFFFFFFFF:64>> == <<255, 255, 255, 255, 255, 255, 255, 255>> would change behaviour, albeit in a correct direction that would match Erlang. This may be desirable, but I've taken the conservative route initially.
   3. See discussion Optimise codegen for constant segments in bit arrays on JS #3724 (comment) for more details.

Performance

Evaluating <<-1:32>>: 10-11x faster.

Evaluating a large lookup table of 16-bit integers: 26x faster.

Pattern matching a 32-bit signed integer: 2.7-7.1x faster, depending on how early a non-matching byte occurs.

Generated JavaScript code size is up to ~70% smaller. There are a few cases when using 40-bit and 48-bit wide integers that can see code size increases of ~20% with certain values, but the average case still sees a code size reduction on the wider integers.

[lpil]

Oooh very nice!!!

I've left some minor notes inline.

When you are ready for a review please un-draft this PR. Thank you!

[lpil]

Only test one thing per test please.

[richard-viney]

Fixed.

[lpil]

Fantastic! Thank you!

Seeing as this is tricky runtime behaviour could you add some integrations tests please 🙏

gleam/test/language/test/language_test.gleam

Line 954 in 31966a0

fn bit_array_tests() -> List(Test) {

[richard-viney]

I've updated the existing language tests so each test is run both as a constant value that is compile-time evaluated, and as a dynamic value that is runtime evaluated.

[lpil]

Amazing work again!!! 💜