lavc: add standalone cached bitstream reader

The cached bitstream reader was originally written by Alexandra Hájková
for Libav, with significant input from Kostya Shishkov and Luca Barbato.
It was then committed to FFmpeg in ca079b0, by merging it with the
implementation of the current bitstream reader.

This merge makes the code of get_bits.h significantly harder to read,
since it now contains two different bitstream readers interleaved with
 #ifdefs. Additionally, the code was committed without proper authorship
attribution.

This commit re-adds the cached bitstream reader as a standalone header,
as it was originally developed. It will be made useful in following
commits.

Integration by Anton Khirnov.

Signed-off-by: Anton Khirnov <[email protected]>