Fix misuse of *usage* in rule identifiers

In the prior commit, we fixed prose occurrences of *usage*.  The same
reasoning applies to the rule identifiers.

These rules describe where the language permits a construct to appear.
On the surface, that may seem to be a kind of pattern of use, but it's
not (in the sense implied by *usage*), and ultimately *use* is still
the right word.  The word *usage* describes conventions and customs --
i.e., *customary* patterns of use.  It describes how a community uses
something and how that varies, e.g., across regions.

That's not what we're doing here.  We're not describing better or
worse Rust idioms.  There's no "nonstandard usage" of Rust grammatical
constructs -- that's not what we're defining, at least.

Let's fix each rule identifier to use `use` instead.

(None of these identifiers were referenced elsewhere in
the Reference.)