Rustdoc documents these with the name of the type alias instead of normalizing them to the underlying type.
Use associated types instead so that the generated docs for nightly-rustc are easier to read.

This should both make the code easier to read and also greatly reduce the amount of codegen
the compiler has to do, since it only needs to monomorphize `create_query_frame` for each
new key and not for each query.

- Disallow multiple macros callbacks in the same invocation. In practice, this was never used.
- Remove the `[]` brackets around the macro name
- Require an `ident`, not an arbitrary `tt`

It's unnecessary.