undeprecates c.parse

We never thought that c.parse is going to be completely subsumed by
quasiquotes, but hoped that the use cases that are going to be lost
aren’t going to be noticed by anyone.

Unfortunately, this isn’t the case, so I’m undeprecating c.parse until
we get a better story for those for whom quasiquotes are not enough.
Merge pull request #3231 from xeno-by/topic/undeprecate-parse

undeprecates c.parse