bytestring.c, catacomb/__init__.py: Compare for equality in constant time.

There's an explicit `ctstreq' function which just does what you wanted.
Also, `ByteString' objects now have a rich-compare method which always
compares for equality in constant time.  Ordering comparisons are
variable time still.

There's a little chicanery to retain the hash function from `str'.

Also add a simple `check' method to `GHash' and `Poly1305Hash' which
compares a hsah or MAC tag in constant time and returns a boolean
result.