Avoid defining MRB_INT16 adn MRB_INT64 at the same time. This is a poka-yoke.
Add a descrption of MRB_INT16.
Merge pull request #1215 from monaka/pr-avoid-define-MRB_INT16-and-MRB_INT64-at-the-same-time

Avoid to define MRB_INT16 and MRB_INT64 at the same time