Generate type numbers at meta-time.

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.

meta/build.py

@@ -5,6 +5,7 @@
 from __future__ import absolute_import
 import argparse
 import isa
+import gen_types
 import gen_instr
 import gen_settings
 import gen_build_deps
@@ -18,6 +19,7 @@ out_dir = args.out_dir
 
 isas = isa.all_isas()
 
+gen_types.generate(out_dir)
 gen_instr.generate(isas, out_dir)
 gen_settings.generate(isas, out_dir)
 gen_encoding.generate(isas, out_dir)