Legalization pattern emission WIP.

Begin emitting legalization patterns in the form of two functions,
'expand' and 'narrow' that are included in legalizer.rs.

The generated code compiles, but it is not fully working yet. We need to
deal with the special cases of instructions producing multiple results.

lib/cretonne/meta/gen_instr.py

@@ -446,16 +446,12 @@ def gen_inst_builder(inst, fmt):
         rvals = ', '.join(len(inst.value_results) * ['Value'])
         rtype = '({})'.format(rvals)
 
-    method = inst.name
-    if method == 'return':
-        # Avoid Rust keywords by appending '_'.
-        method += '_'
-
     if len(tmpl_types) > 0:
         tmpl = '<{}>'.format(', '.join(tmpl_types))
     else:
         tmpl = ''
-    proto = '{}{}({}) -> {}'.format(method, tmpl,  ', '.join(args), rtype)
+    proto = '{}{}({}) -> {}'.format(
+            inst.snake_name(), tmpl,  ', '.join(args), rtype)
 
     fmt.doc_comment('`{}`\n\n{}'.format(inst, inst.blurb()))
     fmt.line('#[allow(non_snake_case)]')