Consider integer dot products

[penzn]

Wasm SIMD has two proposed dot product extensions.

2-dimensional dot product with 16-bit inputs

WebAssembly/simd#127. There are two variants: dot product, and dot product with accumulation (dot+add). The latter would not make it into SIMD MVP and can be considered here.

4-dimensional dot product with 8-bit inputs

WebAssembly/relaxed-simd#9. Those instructions have hardware equivalents in SDOT (Arm) and VNNI (x86), but not necessarily amount the instruction sets targeted by Wasm SIMD proposal.