rustfmt attribute to keep match clauses sorted and merged

When writing some long matches that map one kind of value to another, it can be extremely helpful to keep the clauses in alphabetically sorted order, as well as merging multiple mappings to the same value.

I'd love to have a rustfmt attribute, along the lines of #[rustfmt::sorted], which when applied to a match would cause rustfmt to keep the clauses sorted and merged.

By making this a dedicated attribute, it can't affect code unless someone opts into it, and by opting in, the user is explicitly stating that the order doesn't semantically matter. Thus, rustfmt doesn't have to prove that sorting doesn't change the meaning of the code; it can rely on the user for that.

The issue names no files, tests, or entry points. Start by locating rustfmt's handling of custom attributes and match-expression formatting, then determine how sorted and merged clauses would be specified and validated; done means an opt-in attribute produces the requested ordering without affecting unannotated matches.