Add back debug_nal

[mortie]

As https://github.com/aizvorski/h264bitstream/issues/31 points out, the debug_nal function doesn't exist anymore. I did a git bisect, and found that it was removed in https://github.com/aizvorski/h264bitstream/commit/eba676c485f68a296bf9d423f07ee1d41c09d8cb. This patch just adds the functions back to h264_stream.c, as they were before https://github.com/aizvorski/h264bitstream/commit/eba676c485f68a296bf9d423f07ee1d41c09d8cb, with some changes because some of the fields on various structs have been renamed since them.

[tmm1]

:+1:

[ezequielgarcia]

:+1: looks good to me!

[ezequielgarcia]

@aizvorski Gentle ping.