Removed crypto_memcmp and crypto_zerocmp

Removed crypto_memcmp and crypto_zerocmp

Fixes #38

This breaks compatibility.  Users need to switch to the crypto_verify
functions.  Sorry.

I do not break compatibility lightly.

Under the heaviest optimisation options (-O3), the old comparison
functions generated a huge amount of code, with quite a few
conditional branches.  It wasn't clear those branches weren't input
dependent.  This could lead to timing attacks down the line.

This is not just theoretical.  During my tests, I have observed
suspect timings (that's why I looked at the assembly in the first
place).  I tried to tweak the implementations, to no avail (some of my
tweaks actually made things worse).

Using more reasonable optimisation settings (-O2) is not an option:
the performance of `-O3` is simply too juicy to be ignored.  Some
users *will* sacrifice security to use it, even if I tell them not to.

The crypto_verify functions emit very terse and clean assembly, which
contains no conditional branches, and no input dependent indices.
That I can trust.