Include binding references in syntax properties

[jackfirth]

In order to make things like #185 easier to implement, it would be handy if refactoring rules could look up binding info for identifiers. This ought to be possible by adding augmenting binding identifiers (e.g. the x in (let ([x 1]) _)) in the syntax objects handed to rules with a 'use-sites property. The property would contain a list of identifiers corresponding to usages of that binder.

This won't catch disappeared uses, but in theory we can fix that bridge when we get there by looking for disappeared use properties on the output syntax objects produced during macro expansion.