Using `par_iter::*` is too broad about rayon

[nikomatsakis]

Hmm, yes, I was thinking recently that prelude::* is more the standard pattern anyway.

[nikomatsakis]

I guess the thing to do would be to have pub mod prelude in the root level that imports all the par_iter traits. Probably not worth putting join in there -- I figure people would prefer to write rayon::join(..) (or, at least, I would).

(The main reason to have those modules be public is just so that users can name all the intermediate types if they should want to.)

[cuviper]

Yeah, I think `join` is too general a name to be a prelude import, and `rayon::join` is easy enough. The `par_iter` traits alone would be a great start for this.

[nikomatsakis]

I wonder if it would be helpful to have the prelude include rayon itself, so that one can write:

use rayon::prelude::*;
...
rayon::join(...)