aac::parser_c: support non-byte-aligned ADTS headers

This is currently theoretical only as all ADTS headers without the
emphasis field are byte-aligned. I might re-introduce support for the
emphasis field again one day, and for that it may be better to have this
mostly correct in advance.

Note that only the raw AAC-ADTS reader supported the emphasis bit
anyway. Neither of the other readers (AVI, FLV, Matroska, OGM, MP4,
RMFF, MPEG-TS) did.

The usage of bc.get_bytes() instead of a simple memory_c::clone() won't
impact performance as bc.get_bytes() uses std::memcpy() directly for
byte-aligned reads.
AAC packetizer: use aac::parser_c
AAC packetizer: support partially-timestamped, unframed packets
AAC common: remove unused functions after conversion to aac::parser_c
AAC common: resolve variable shadowing
AAC common refactoring: move functions to aac namespace
AAC common refactoring: move aac_header_c to aac namespace; remove globals
mkvinfo: track stats: use correct variable for max timecode calculation

Fixes #1092.