[enhancement] impl should support type alias

### What's hard to do? (limit 100 words)

Currently it's a bit cumbersome to define derivative types closely related to another a given struct parametrics.

### Current best alternative workaround (limit 100 words)

One need to derive type from parametrics on every use thru local `type =`.

### Your view of the "best case XLS enhancement" (limit 100 words)

Similar to `const` support within impl, it should be nice to be able define struct-level type aliases that are in scope within impl methods.