Detect unused #[deftly] attributes

Users who misspell or misdirect these attributes don't get any kind of warning. This is not great.

We can't produce warnings, but we could produce errors. I think I can see how to do this for precanned templates. I don't think it is possible where truly-adhoc templates are involved.

Therefore any fix would make it necessary to declare on the driver whether truly-adhoc templates will (or may) follow, something like this perhaps:

#[derive(Adhoc)]
#[derive_adhoc(later)]

This would be a breaking change but I think that's tolerable.

I'm not sure about the word later. Do we need a term for truly-adhoc templates? Other options would include a separate helper attribute eg #[derive_adhoc_later]. Or some punctuation (I don't much like this).

My proposed semantics are to produce an error if the driver has #[adhoc(foo(bar)) unless any of the invoked templates mentions Xmeta(foo(bar)) (as expansion or condition) for the appropriate X, anywhere in the template. (So it wouldn't depend on whether that part of the template was expanded; if that's a problem for a tamplate with complex attributes, it would be for the template to deal with.)