Make MakeCompoundType package private (#1304)

And same for MakePrimitiveType. This also makes sure that we return
the same primitive type every single time.

Fixes #1271

types/string.go

@@ -38,7 +38,7 @@ func (fs String) ChildValues() []Value {
 	return nil
 }
 
-var typeForString = MakePrimitiveType(StringKind)
+var typeForString = StringType
 
 func (fs String) Type() *Type {
 	return typeForString