eina: improve cmp function of eina_hash string related hash.

For strings, we already know the length, so let's just not call the
heavy strcmp if the length is not already the same. Get some easy
speedup here.

Backport from commit ff845b0a7c9bb6de48f5786a6a8a092d8819debf
on https://git.enlightenment.org/core/efl.git