We need to generate hash tables keyed by types, so the Python scripts need to
know the index used to represent types in Rust code.
To enforce this, add a new gen_types.py script which generates constant
definitions for the ir/types module.
Also generate constants for common SIMD vector sizes.
Amend build script to generate an encodings-<isa>.rs file for each target ISA.
Emit a function that can evaluate instruction predicates.
Describe the 3-level tables used for representing insrruction encoding tables.
Add Python classes representing the tables.
The generated code is incomplete and not used anywhere yet.
Clarify terminology by always referring to a 'Target ISA' instead of just
'Target'. Use 'isa' as a module name instead of 'target' both in Rust and Python
code.
This is only to clarify terminology and not at all because Cargo insists on
using the 'target' sub-directory for build products. Oh, no. Not at all.
