Release 1.1.3

This is a minor release focusing mainly on optimizations and bug fixes.
Changes include:
- Neon optimizations inproving performance on ARMv7 and ARMv8 by up to 15%
- Fixes some issues with 16-bit platforms (e.g. TI C55x)
- Fixes to comfort noise generation (CNG)
- Documenting that PLC packets can also be 2 bytes
- Includes experimental ambisonics work (--enable-ambisonics)
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