Add SIMD instructions to syntax

[ngzhian]

Main changes are defining:

• ixx for i8x16, i16x8, i32x4, i64x2
• fxx for f32x4, f64x2
• vxx for ixx and fxx
• the the rest of the SIMD instructions are defined based on these

Unfortunately, the definition of instructions aren't as clean as the existing ones, due to high asymmetry (see https://github.com/WebAssembly/simd/blob/master/proposals/simd/NewOpcodes.md for an overview).

This leads to a lot of specific i8x16, i16x8, i32x4 instructions defined (since i64x2 has to be left out).

I also tweaked the paragraph describing comparisons, to indicate that comparisons can return a boolean, or return a value of the same type, because the SIMD comparisons return S128. I'm not sure if this tweak should stick, we can also classify SIMD comparisons as Binary instructions, but that's confusing too. I guess I don't know if these subcategories are meant to classify instructions by "the kind of things they do" or "the type signature"?

See https://www.ngzhian.com/simd/core/syntax/instructions.html for a preview.

[rossberg]

Some suggestions for separating specification from explanation.

[ngzhian]

I noticed that some instructions that take immediates were not properly specified:

• v8x16.shuffle
• extract_lane
• replace_lane

Introduced a new production laneidx, which is byte, and added it to the appropriate instructions. see 18049c7.