Add scopes field to `expect-syntax`

[jackfirth]

It should accept a syntax object that the input syntax should have the same scopes as. If the syntax-e argument to expect-syntax is a syntax object, this can default to that.

[jackfirth]

This can be done by taking a gensym and constructing two identifiers with it using the two different syntax objects for their lexical contexts, then comparing those identifiers with free-identifier=?.

[jackfirth]

Correction: use bound-identifier=?, not free-identifier=?. Duh.