Move ValueType into a new cdsl top-level module.

We want to separate the Python classes that make up the DSL used to
define the Cretonne language from the concrete definitions.

- cdsl.types defines the ValueType class hierarchy.
- base.types defines the concrete types.

lib/cretonne/meta/cdsl/__init__.py

@@ -0,0 +1,6 @@
+"""
+Cretonne DSL classes.
+
+This module defines the classes that are used to define Cretonne instructions
+and other entitties.
+"""