crypto: drop pretense of notmuch_crypto_context_t

crypto: drop pretense of notmuch_crypto_context_t

notmuch_crypto_context_t was introduced (i think) as some sort of
abstraction layer to make notmuch somewhat independent of GMime.  But
it isn't even useful for GMime 3.0 or later -- we can drop the
pretense that it's some sort of abstraction in this case, and just
call it what it is, GMimeCryptoContext, which is useful for building
against older versions of GMime.

This also renames _notmuch_crypto_get_context() to
_notmuch_crypto_get_gmime_context().