Get the Type instance corresponding to a given type

The result of `type_as_value x` is the same as `x.type`.

Internally, Fuzion's front end implements `x.type` using
`type_as_value x`. The middle end then replaces calls to
`type_as_value` by `T`'s type clazz.
The type clazz is then used to call the correct type feature.