Opus release 1.3-rc

This is a release candidate for the upcoming 1.3 release and includes:
- Making it possible to use SILK down to bitrates around 5 kb/s
- Using wideband encoding down to 9 kb/s
- Improving security (including a new --enable-hardening option)
- Minor quality improvement on tones
- Improving Ambisonics support (still experimental)
- Minor bug fixes
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