Constant time equal function on `string`

From some crypto libraries like digestif or Callipyge, it needed to have a constant time equal function to avoid timing attacks. To avoid replication of code and ensure maintainability of this kind of function, we decide to provide a little package which implements equal function on string.

This package provides 2 implementations (a C implementation which comes from OpenBSD's code and an OCaml implementation which is simply a translation of the C code) well tested to check constant time assumption (see tests).

Happy hacking!