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/base/__init__.py

@@ -0,0 +1 @@
+"""Definitions for the base Cretonne language."""