8k, 12k and stereo are mostly untested

So it can be used outside of the SILK PLC

Brings the DRED implementation in line with the -00 draft

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

Everything can now be shallow-copied again

Makes shallow copy of decoder possible again

progressive decode, avoid storing DRED decoder state

Adding a 'D' byte to signal the DRED experiment, along with a version
number byte. This entire commit will be reverted once DRED is finalized
and given a non-experimental extension number.

Properly re-initialize DRED (only) when needed

Probably still has many bugs