Support references to generic types in quoted code

[shadaj]

fold initialization with Default: better error messages

I was puzzled by the fact that my KVS<V> that stores V's was compiling but failing at runtime due to

.fold(q!(|| V::default()), q!(|acc, i| *acc = i));

Turns out that Claude knew that I could use Default::default():

.fold(q!(|| Default::default()), q!(|acc, i| *acc = i));

That makes sense to me based on what I know about q!, but it'd be helpful to have better messages than:

error[E0433]: failed to resolve: use of undeclared type `V`
   --> examples/_process_0_549E49F2.rs:852:48
    |
852 | ...                   || V::default()
    |                          ^ use of undeclared type `V`
    ```

_Originally posted by @jhellerstein in https://github.com/hydro-project/hydro/discussions/2223#discussioncomment-14808784_