Fix the handling of special types in type variables.

- Allow the syntax "specials=True" to indicate that a type variable can
  assume all special types. Use this for the unconstrained type variable
  created in ast.py.
- Fix TypeSet.copy() to avoid deepcopy() which doesn't do the right
  thing for the self.specials set.
- Fix TypeSet.typeset_key() to just use the name of special types
  instead of the full SpecialType objects.

lib/cretonne/meta/cdsl/ast.py

@@ -247,7 +247,8 @@ class Var(Atom):
                     'typeof_{}'.format(self),
                     'Type of the pattern variable `{}`'.format(self),
                     ints=True, floats=True, bools=True,
-                    scalars=True, simd=True, bitvecs=True)
+                    scalars=True, simd=True, bitvecs=True,
+                    specials=True)
             self.original_typevar = tv
             self.typevar = tv
         return self.typevar