[WIP] Replace rayon with WIP latch-target-thread branch.

[tdaede]

[jamrial]

Wouldn't it be safer to include a known good snapshot of this branch inside the crates folder, and use that instead of a remote private branch that may change in unpredictable ways?

Also, https://doc.rust-lang.org/edition-guide/rust-2018/cargo-and-crates-io/replacing-dependencies-with-patch.html mentions that [patch] should be used instead of [replace].

[coveralls]

Coverage remained the same at 75.614% when pulling f27967f8f6e599d821d8e458ea699384ae2930be on tdaede:rayon_replace into 9e1c71108121f5b48a30337cd1bd8985fc3e3250 on xiph:master.

[tdaede]

Yes, sorry I forgot to mark this WIP. I'd rather figure out why this is the case (the rayon branch is not supposed to improve performance, just reduce cpu utilization of idle threads) and then get a full release of rayon.

[lu-zero]

With this patch on top of 7344daee25bc94620d7ac6577d1905089fab27f6, Bosphorus 4k 8bit.

4x4 tiles on a 8 threads i7:

encoded 600 frames, 0.292 fps, 5481.45 Kb/s

7344daee25bc94620d7ac6577d1905089fab27f6 alone:

encoded 69 frames, 0.279 fps, 6129.46 Kb/s