The unbounded fractional camera

See merge request FP/iris-coq!195

- More consistent indentation.
- Mark new subgoals as comments.

Lemmas about subst_map on closed expressions

See merge request FP/iris-coq!194

Two small lemmas about binder_insert

See merge request FP/iris-coq!193

This is to avoid confusion with those of selection patterns.

This lemma is similar to `later_ownM`.

This lemma allows one to get the witness out of a later, without having
to use `later_car`, i.e. in a way that works in the on-paper version of
the logic.