Reintroduce rayon via `Output` trait

[rphmeier]

Vague description in #34 (comment)

[darrenldl]

@rphmeier I have been thinking if the use of rayon can be replaced with parallelism constructs which are available in no-std environment.

Are you aware of anything of that sort?

[rphmeier]

I'm not aware of anything that's available generically in no-std. For now I think it would be best to implement a WithRayon adapter available under std, perhaps even under a rayon feature.

[darrenldl]

Ah gotcha. Yeah that makes sense, I was just curious if parallelism can be done in no-std environment for speed up.

[darrenldl]

Put on hold until #48 is resolved.