String: performance improvement converting uint64_t to string (4x)

Based on the trick given by A. Alexandrescu on Facebook Engineering
blog, this patch replace the old mk_str_itop() with the faster
algorithm proposed.

More details about the implementation here:

 https://www.facebook.com/notes/facebook-engineering/three-optimization-tips-for-c/10151361643253920

Signed-off-by: Eduardo Silva <eduardo@monkey.io>