Finding the best codebook involves comparing different paths, where each
path is a sequence of several decisions (namely which codebook to use).
Up until now, these sequence was encoded in a NUL-terminated string and
the actual decisions were encoded as ’\0'..'\3' (which encoded 0..3).
This commit modifies this to actually encode it via 0..3 by switching
away from a C-string to a simple array with an explicit length field.
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>