Shorten `Associated` to `Assoc` and rename `Method` to `AssocFn`.

[eddyb]

We've been moving away from the "static method" terminology, so IMO we should be more uniform with all associated items, and treat the general case of "methods that may or may not have self" as simply "associated functions".
"Method" terminology would then remain primarily for "method call", i.e. dot syntax for asymmetrically providing the self argument.

Because AssociatedFn is a bit long, I'd find it fitting if we shortened Associated, across rustc, to Assoc, so we end up with AssocFn (which only one letter longer than Method).
Also, when Method appears in the context of associated items already (e.g. {Trait,Impl}ItemKind), it can be just Fn.

cc @rust-lang/compiler (and in particular, I'm curious what @nikomatsakis thinks the decision process for "cosmetic" changes like these should be)

[nikomatsakis]

I think that discussing terminology like this is probably a good fit for a design meeting. I also think this particular change is a good idea.

[nikomatsakis]

I've added this to my temporary list of design meeting ideas

[agnxy]

Hi @eddyb @nikomatsakis I'd like to work on this if the decision has been made.

[oli-obk]

@agnxy one thing that doesn't need a decision is to change all things named Associated... to Assoc.... So you can do that already.