aacenc: Move small misc. functions to a separate file

As well as tables littered everywhere, functions were spread
out all across the encoder's files. This moves them to a single
place where they can be used by either the encoder's main files
or additional encoder files. Additionally, it changes the type
of some to 'inline' to enable us to simply put them in a header
file and possibly gain some speed due to compiler optimizations.

Signed-off-by: Claudio Freire <klaussfreire@gmail.com>