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/check.sh

@@ -14,7 +14,7 @@ runif() {
 # Check Python sources for Python 3 compatibility using pylint.
 #
 # Install pylint with 'pip install pylint'.
-runif pylint --py3k --reports=no -- *.py cretonne isa
+runif pylint --py3k --reports=no -- *.py cdsl base cretonne isa
 
 # Style linting.
 runif flake8 .