[enhancement] impl should support type alias

[proppy]

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.

[proppy]

Even better if those would also be visible in struct definition so that you can use alias to type members related to struct parametrics.