Migrate AVIF encode to gamut-avif; delete libaom and ravif

[justin13888]

• Replace ravif (avif-encode-ravif) with gamut-avif for Rgb8 lossless now.
• 10/12-bit, alpha, and lossy input surface: blocked slice on gamut-avif: encoder support for high-bit-depth and non-Rgb8 inputs gamut#251 — per the no-interim policy, libaom-sys/avif-serialize/aom-bindgen and all avif-encode-libaom* features are deleted in this issue, with high-bit-depth AVIF encode temporarily unavailable and documented as such.
• Drop deps: ravif, libaom-sys, avif-serialize (+ aom build.rs slice).
• Ref Implement space-efficient AVIF encoder gamut#170/#171 for quality work.
• Depends on: rawshift-core: replace generic primitives with gamut re-exports, keep sensor types #18, Migrate metadata stack to gamut-metadata #19.