Remove std feature from rayon feature

[nazar-pc]

I found no good reason for rayon feature to require std. In fact it'd be nice to just use rayon all the time sometimes regardless if it is used in std or no_std environment.

UPD: Looks like even rayon-core depends on std internally (which may or may not change in the future), but there is still no direct need for it in BLAKE3 itself.

This complements #469

[nazar-pc]

cc @oconnor663, not sure if you're getting notifications about Rust changes without explicit pings

[oconnor663]

I think you're right on both counts. I don't think it's possible to pull in rayon without also pulling in std, but also there's no particularly reason we need to do this explicitly, and it's cleaner not to. Thanks!