Constant-Time bin2hex() implementation

Constant time bin2hex() implementation, taken from https://github.com/jedisct1/libsodium/commit/814df1e60d282fd0f52f53e0ba38c2e252f436dd

Since Rasmus & co do not understand timing attacks (in particular: cache-timing attacks), as evidenced at http://news.php.net/php.internals/79190, I'm submitting it also to Hacklang and not just PHP. May the best team win.