[dslx] typeof / struct field type references

There should be a way to reference the type of a struct field. For example:

```
struct Foo {
bar: u3,
}
let foo: Foo = ...;
let bar: Foo::bar = foo.bar;
// bar has type u3
```

Without this feature, we often accidentally couple definitions to explicit types, and it's unreasonable IMO to ask people to create named type aliases for each and every field in a massive codebase.