Controls whether deep PLC is enabled

No dependency on the data files if no DNN code enabled

Signed-off-by: Jean-Marc Valin <jmvalin@amazon.com>

Signed-off-by: Jean-Marc Valin <jmvalin@amazon.com>

When ready, change DRED_EXTENSION_ID to the final ID, remove
DRED_EXPERIMENTAL_VERSION completely, and change DRED_EXPERIMENTAL_BYTES
to zero (eventually remove it).

Should work for both CBR and VBR. In the CBR case, we can make
CELT VBR and use DRED to fill the rest.

If configuring CELT for CBR but controlling the bitrate with
OPUS_SET_BITRATE rather than nbCompressedBytes, then the range
coder buffer would never get resized. AFAICT this could never
be triggered in Opus because CBR was also controlled by
nbCompressedBytes.

Avoids interactions with redundancy settings

SET_BITRATE is now the total bitrate again

We weren't reserving enough bytes for the DRED extension

Also, don't code DRED that's redundant with the main packet

Should make synthesis easier in the future

So it can be used outside of the SILK PLC

output() renamed to decode(), dred objects using alloc() and free(),
OpusDRED now passed as cost for decoding.