Implement i32/i64/f32/f64.add etc. inline-assembler builtins

[dcodeIO]

As of today, there are various inline-assembler-like builtins implementing the WebAssembly instruction set, but these only cover instructions that cannot be represented by a simple unary or binary expression, for example there is no i32.add(x, y) and one would instead write x + y. The usefulness of these is arguable therefore, but there is one compelling case where having these would enable something not possible otherwise, namely i64.add and friends to polyfill i64 support in the portable library (see also).

Implementing these is mostly an easy matter of copy-pasting from already supported instructions, but it's time consuming due to updating all of src/builtins.ts, std/assembly/builtins.ts and std/assembly/index.d.ts with a lot of instructions if we implement all of them, which I'd prefer. Pinning here as a good first issue, and so we don't forget :)

[dcodeIO]

For completeness, and to track progress, here is a (hopefully) complete list of the missing inline-assembler-like builtins:

Integers and floats

• [x] add
• [x] sub
• [x] mul
• [x] div (int has unsigned variant)
• [x] eq
• [x] ne
• [ ] lt (int has unsigned variant)
• [ ] le (int has unsigned variant)
• [ ] gt (int has unsigned variant)
• [ ] ge (int has unsigned variant)

Integers only

• [x] rem (has unsigned variant)
• [ ] and
• [ ] or
• [ ] xor
• [ ] shl
• [ ] shr (has unsigned variant)
• [ ] eqz (unary)

Floats only

• [ ] neg