Adds constant time equal functions for strings and bytes

[janilcgarcia]

Hoping to help with #13 to avoid timing attacks on hashes and other comparisions.
I'm not sure how to write tests to make sure the functions are constant time, but I'm pretty sure they are, please review, tho.

[wiml]

If there is a difference in the middle of the string, then for bytes after that point, eq will no longer be called (because && is a "short-circuiting" operator). This will be a small difference, but may still be detectable with high resolution timing attacks or cache-line shenanigans.

[xavierleroy]

Thanks for taking a stab at this problem. Here is an implementation, based on @wiml's comment in #13, that should be constant-time:

let seq_equal (len: 'a -> int) (get: 'a -> int -> char) (s1: 'a) (s2: 'a) =
  let l = len s1 in
  let rec equal i accu =
    if i >= l
    then accu = 0
    else equal (i + 1)
               (accu lor ((Char.code (get s1 i)) lxor (Char.code (get s2 i))))
  in
    l = len s2 && equal 0 0

let string_equal = seq_equal String.length String.get
let bytes_equal = seq_equal Bytes.length Bytes.get

[xavierleroy]

I went ahead and committed the implementation above.