[enhancement] DSLX Polymorphism

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

It's hard to write reusable components like an arbiter because you ideally want it to be type-generic, e.g.

```
proc Arb {
...
config(inputs: chan[N] in, outputs: chan out) { ... }
...
}
```

With traits (#617, #618), you could event take advantage of interfaces to do more interesting things besides shuffling data around.

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

Rewrite or use templating. Channel legalization builds type-generic arbiters as part of an opt pass, which is an option if the pattern is common enough.

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

Rust-style type generics with traits.