Allows us to exclude the most recent silence from DRED

Adjust q0, qD and duration based on bitrate and loss.

Saves ~270 kB of weights in the decoder

Only code non-degenerate symbols, which makes the encoder faster

Using a max lambda of 0.04

Remove old pitch estimator and retrain all models

More efficient than PVQ

There was a suggestion to use 127 for extending the extension ID space

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 make synthesis easier in the future

Brings the DRED implementation in line with the -00 draft

Everything can now be shallow-copied again

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.