Merge branch 'move-global-func' into 'master'

Move global functor to base_logic

Sorry to bring this up again, I just think it's important to get this as right as we can, right now. ;)

This moves the global functor and dynamic composeable ownership to `base_logic`. The reasoning behind this is that if someone builds another program logic in our base logic, they will likely want to re-use this, so it makes more sense to have it in base_logic than in program_logic.

The rule for stuff to be in program_logic is: It depends on the Iris' notion of a language, or on world satisfaction (i.e., on our specific way to do invariants). Thus, `saved_prop` also moves to `base_logic` and becomes reusable by other program logics.
(If we want, we can talk about changing this to just "anything that depends on our notion of a language". Is it conceivable that our world satisfaction is reusable?)

When we talked about this previously, Robbert raised concerns about the name `iProp`. However, considering that we also have `iTactics` and use `I` for the `uPred_scope`, I feel like the letter "i" has already leaked way beyond `program_logic`, so this PR doesn't actually change anything. If you want, interpret `iProp` as "internal propositions", that even makes sense. ;)

Cc @robbertkrebbers @jjourdan

See merge request !18