Begin generating code for the legalizer.

This is a work in progress. The 'legalizer.rs' file generated by
gen_legalizer.py is not used for anything yet.

Add PEP 484 type annotations to a bunch of Python code.

lib/cretonne/meta/build.py

@@ -10,6 +10,7 @@ import gen_instr
 import gen_settings
 import gen_build_deps
 import gen_encoding
+import gen_legalizer
 
 parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
 parser.add_argument('--out-dir', help='set output directory')
@@ -23,4 +24,5 @@ gen_types.generate(out_dir)
 gen_instr.generate(isas, out_dir)
 gen_settings.generate(isas, out_dir)
 gen_encoding.generate(isas, out_dir)
+gen_legalizer.generate(isas, out_dir)
 gen_build_deps.generate()