A constant-time version of memcmp(), useful to compare nonces and counters
in little-endian format, that plays well with sodium_increment().
Unlike sodium_memcmp() which can compare anything for equality,
sodium_compare() is designed to compare things that are comparable, byte by
byte. Therefore, the prototype is slightly different: its arguments are
supposed to be `const unsigned char *`.
The names sodium_memcmp() and sodium_compare() are slightly confusing.
But we're not going to rename sodium_memcmp(), and I cannot think of a
better name for sodium_compare() than sodium_compare().
