This was more subtle than I expected...

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

lifetime logic: use agree instead of dec_agree

If you are happy with this, we can merge this and <https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/22>

Cc @robbertkrebbers @jjourdan

See merge request !4

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.