Execution semantics (#340)

- Add execution semantics for SIMD instructions.
- Add new numeric primitives required for SIMD instructions.
- Rename auxiliary lanes function to dim (number of elements in a shape)
- Define auxiliary lanes function to interpret i128 as a sequence of numeric values
- Define auxiliary saturation operators, and use it for existing trunc sat
- Fix definition of trunc and convert in SIMD instruction syntax, pull them out into their own production, and add validation for them.

Author: ngzhian