Create a RecType, normalizing its contents. This means:
Create a RecType, normalizing its contents. This means:
Nested Rec types on the type's spine are merged with the outer one.
Any refinement of the form type T = z.T on the spine of the type
where z refers to the created rec-type is replaced by
type T. This avoids infinite recursions later when we
try to follow these references.
TODO: Figure out how to guarantee absence of cycles
of length > 1