[Pending RFC] Const functions and inherent methods.

[eddyb]

[DO NOT MERGE] RFC is rust-lang/rfcs#911.
r? @nikomatsakis

[bors]

☔ The latest upstream changes (presumably #22796) made this pull request unmergeable. Please resolve the merge conflicts.

[eddyb]

I've had to remove the commit using const fns at stage1 and later because I forgot #[cfg(not(stage0))] won't save me from parse errors.
It's a bit sad that I did all that annotation work for nothing - after a snapshot it can be done with no stage0 compatibility concerns, though I'll likely reuse the implementation and just remove the #[cfg(stage)] parts.

[bors]

☔ The latest upstream changes (presumably #22801) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #23298) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #23473) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

The RFC has been accepted now. Sorry that took so long. There was this little issue of a 1.0 beta release; kind of distracting. Anyway, @eddyb indicated on IRC that he wont' have much time lately, so I plan to adopt and rebase this PR.

[nikomatsakis]

I'm going to close this PR. I have a rebased version locally that I will hopefully open soon (just have to resolve one or two minor errors).