Change recursive domain to `proto = 1 + (action *  (V → proto → PROP))`.

See merge request iris/actris!7

By moving the later further to the outside, we can kill many occurences
of `Next`, and get rid of laters/except_0s at awkward positions. Also we
can have a sensible eliminator `proto_elim`.

Session notation

See merge request iris/actris!6

n-ary choice

Closes #3

See merge request iris/actris!5