Types -- sequence of `Type` that may be processed without being boxed

This inherits from `Sequence Type` for convenience of all the
infrastructure provided by `Sequence`. Nevertheless, the
preferred use is via calls to `Types.type_foldf` only since
these calls do not require boxing of the types and permit
code specialization of the actual types.

This is a parent of `Open_Types`, which itself is the parent of
the types generated for calls to open type parameters.