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.

TODO : functions, sums. But these types need to be redefined properly.

Warning : splitting a own of a product only works if the list of types is non-empty.

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

This reverts commit 3ddb6014.
I did not intend to push this to master, sorry :/

Also, perform some refactoring.