Add support for generic attributes

[d0nutptr]

Currently, attributes require concrete types in their definition. This means that if we want two or more structs to be able to satisfy a particular attribute we need to use dacquiri's multiple attribute function support.

This works so long as we know all of the supported types ahead of time. In cases that we're building a library with dacquiri, that may not always be true. It would be better to support generic objects that satisfy some trait bounds.

This was validated in a test that it is feasible.