Matching redeclarations

[zygoloid]

Require exact syntactic matching in redeclarations. Provide new terminology for
redeclaration matching and agreement. Specify non-redeclaration rules for the
other contexts where we require multiple declarations to match, such as impls
of interfaces, impls of virtual fns.

[jonmeow]

I had a couple questions about extern handling as mentioned here. I skimmed through everything else and 👍

[chandlerc]

Largely looks good. Mostly cosmetic comments on my end except for the thunk part for impl functions. But even that is really just trying to find a somewhat cleaner model, not suggesting shifting direction.

[chandlerc]

Sorry this somewhat fell off my radar, I think there's mostly just a wording/nuance tweak that I'd like to try to get working before landing still...

[chandlerc]

LG, shipping it!