This also needs changes in type and continuation context inclusion.
It allows us to prove that the empty sum is equal to the empty type.
I also took the opportunity to rename TCtx_holds to TCtx_hasty, which at least says what is "holding".

I also changed the spec : this can be done even if the lifetime is not
ongoing, and thus it does not need tokens.

Also, to fix the build: use compatibility layer for ownP.
TODO : connect the heap invariant directly.

Refactoring the type system : every type gets its own file. Some trivial ones are defined somewhere else where it make the most sense.