Add specs on rb_fix2int and rb_fix2uint.
Implement rb_fix2int and rb_fix2uint.
Define FIX2INT, NUM2INT, FIX2UINT, NUM2UINT differently when SIZEOF_INT < SIZEOF_LONG.
Merge pull request #2699 from razielgn/capi-fixes

Fix FIX2INT and FIX2UINT C-APIs.