Add InstructionFormat.imm_fields.

Consolidate the imm_members and imm_kinds into this list so the
FormatField is the single definition of these properties.

This makes it easier to access the precomputed FormatFields
parametrically, avoiding going through getattr().

This is better for type checking too.

lib/cretonne/meta/gen_encoding.py

@@ -90,7 +90,7 @@ def emit_instp(instp, fmt):
     fnames = set()  # type: Set[str]
     for p in leafs:
         assert isinstance(p, FieldPredicate)
-        fnames.add(p.field.name)
+        fnames.add(p.field.rust_name())
     fields = ', '.join(sorted(fnames))
 
     with fmt.indented('{} => {{'.format(instp.number), '}'):