RFC for static generic parameters

[rust-highfive]

This is a rough draft to see whether something like this should be done. I'd appreciate feedback on how the technical details could look like, since I do not know a lot about the compiler internals myself.

Tracking issue for postponed PR #56

[Gankra]

CC me

[milibopp]

I rebased my working branch for #56 onto the current master. We could use this as a starting point for discussion.

[milibopp]

I've updated and extended the RFC on my branch a bit. Feedback welcome!

[globin]

CC me

[milibopp]

I posted my current draft on discourse to get some feedback.

[scottmcm]

With #2000 merged; I think this can now be closed.