perf(web): fast path for utf8 decoding - part 1

[littledivy]

• Avoid copying buffers.

https://encoding.spec.whatwg.org/#dom-textdecoder-decode

Implementations are strongly encouraged to use an implementation strategy that avoids this copy. When doing so they will have to make sure that changes to input do not affect future calls to decode().

• Special op to avoid string label deserialization and parsing. (Ideally we should map labels to integers in JS)
• Avoid webidl Object.assign when options is undefined.

Upto ~1.8x improvement on small data.

# This patch
time 1893 ms rate 5282620
time 2067 ms rate 4837929

# main
time 3284 ms rate 3045066
time 3252 ms rate 3075030

[lucacasonato]

This change should be made into a more general change in the WebIDL dictionary converter. That way it automatically applies to all WebIDL dictionary converters, not just this one.

[aapoalas]

Double new Uint8Array(newUint8Array(input))?

[ghost]

Which is a copy... but arguably not too obvious?

Wait, mb, double copy - that's odd.

I'd think it's expecting input: SharedArrayBuffer, so .buffer is false, then constructs an TA in the second branch, then copies it.

[littledivy]

Revived by #16593