Where possible, we should move JavaScript to native/wasm code

This is meta-issue for ongoing work for move as much code as possible out of JavaScript
and into C/C++.

In particular it would be good to focus of code that does HEAP access from JavaScript.
Doing this instead from native code has several advantages:
- The syntax for heap access in JS much harder to read and write
- JS heap access is currently slow when using `ALLOW_MEMORY_GROWTH` and `USE_PTHREADS`
- Supporting wasm32 and wasm64 with the same JS code adds complexity (#12869)