By gop parallelism

[lu-zero]

This set will add by-gop processing and by-gop parallelism to rav1e.

• Use the channel-api to replace the current API (this makes rav1e wasm-incompatible)
  • Make the RC data emission in sync with the packets by making bundles
• Simplify the internals to process the data passing around a Gop structures instead of the current internal accounting and clean up phase
• Use channels to connect the Gop processing stages
• Add simple parallelism by dispatching Gops to multiple pipelines and emit the packets in order from a reordering queue
• Make a new lower level public API for consumers such as rav1e-by-gop to provide richer experience (e.g. multi-node processing)
• Use the lower level API to support the wasm target again.

[lu-zero]

Otherwise implemented and landed :)