Compare two strings in constant-time.
In practice, you almost always want to use hash_equals instead, since it provides the same logic but returns a bool instead of an int. However, if you're using the return value of a comparison in a calculation that's timing-sensitive, and worried about timing leaks with bool-to-int conversions, sodium_memcmp is an ideal replacement.
String to compare
Other string to compare
Returns 0 if both strings are equal; -1 otherwise.