cabac: split cabac.h into declarations and function definitions

      This fixes standalone compilation of some decoders with --disable-optimizations.
cabac.h defines some inline functions that use symbols from cabac.c.  Without
optimizations these inline functions are not eliminated and linking fails with
references to non-existing symbols.

Splitting the inline functions off into their own header and only #including
it in the places where the inline functions are used allows #including cabac.h
from anywhere without ill effects.