Split const generics in two and document inferred consts

[BoxyUwU]

I split some stuff out of the "const parameters" chapter because it seemed weird to talk about types in a different place from consts like we were previously. This PR assumes that feature(generic_arg_infer) has been stabilized and so documents _ arguments to const parameters, so should not be merged until then.

I don't really know how these r[...] annotations are supposed to work. Hope I didn't mess anything up there

[BoxyUwU]

Something I left out here is that the "concrete expressions" are not required to be quite so concrete for repeat expr counts as a backwards compatibility FCW. Does the reference intend to document the actual state of the compiler, or can I take liberties in documenting things as if FCWs are already hard errors?

[traviscross]

@ehuss and I are reviewing this on a call. This is a bit tough to review in that the content was both moved and changed in one commit. If possible, it'd be helpful to us if this could be broken out into (at least) two commits; one to move the content, then the other or others to make the changes and additions.

@rustbot author

[BoxyUwU]

Sure, done

[BoxyUwU]

@rustbot ready

[BoxyUwU]

I don't really know how to use this grammar,types syntax to explain that you can do (((_))) too

[BoxyUwU]

@ehuss @traviscross any chance y'all will be able to get around to reviewing this soon? The stabilization PR is now ready to be merged and it'd be unfortunate to block a relatively minor feature that's already been waiting years, on this PR.

[BoxyUwU]

Alternatively I could file a separate smaller PR that just adds a small extra bit of wording to the existing section noting that _ can be used as an argument to const parameters. Then leave this more involved clean up/rework of const generics handling in the reference off of the "blocking" path for feature stabilization.

[ehuss]

If possible, I think it would be best to drop the rearrangement commit. This probably isn't the organization we'll land on, and I would prefer to keep these as separate concerns so they don't block one another.