Fix multistream packet corruption, implement GET_FINAL_RANGE for multistream, and add many tests.

Multistream encode was failing to add the length of the extra length for
self-delimited packets causing corrupted output. Multistream decode was
not properly handling lost frames (and potentially reading out of bounds
as a result).

GET_FINAL_RANGE has been implemented as the xor of the final range of all
the streams in the packet.

test_opus_encode now does the mono narrowband tests using dual-mono
multistream.