nmigen.lib: Add RoundRobin

[jeanthom]

Fixes #448

[jeanthom]

Currently implementing @jfng's suggestions (code styling, better doc, etc.).

[whitequark]

I feel like this should be elaborated. Something like (phrasing needs to be improved):

For a given set of requests, the round-robin scheduler will grant one request. Once it grants a request, if any other requests are active, it grants the next active request with a greater number, restarting from zero once it reaches the highest one. Use :class:EnableInserter to control when the scheduler advances.

[jeanthom]

I tweaked a little bit what you wrote. What do you think?

[whitequark]

Great, thanks for the work! There's only one remaining concern I have: should RoundRobin live in its own module? Do we plan to have other schedulers? Or perhaps should it be categorized differently?

[jeanthom]

That's the way it was in omigen because there was only one scheduler. I seriously doubt that we're going to have other schedulers in nMigen, but just in case we could name the module scheduler? Or keep it this way until someone has another scheduler to offer?

[whitequark]

I seriously doubt that we're going to have other schedulers in nMigen, but just in case we could name the module scheduler? Or keep it this way until someone has another scheduler to offer?

I think we can name the module scheduler and at the same time accept that there likely won't ever be a second one. The module name is still nice because from nmigen.lib.scheduler import RoundRobin is more clear on its purpose than from nmigen.lib.roundrobin import RoundRobin.

[whitequark]

Thanks!