Skip emitting a transposed type if the original type is entirely generic

[ealmloff]

The transpose operation on stores pushes the reactivity one layer deeper into the type. For example, transposing Store<Result<T, E>> gives you Result<Store<T>, Store<E>>.

The store macro creates a transposed version of every type with the fields replaced with their store variants. For types similar to Result and Option with only generic fields, the transposed version of the type is more difficult to name during matching.

This PR adjusts the macro logic to only emit a type alias all fields of the item we are deriving store on are generic. This makes it possible to match on the transposed version of the item as normal:

#[derive(Store)]
#[non_exhaustive]
enum Enum<T> {
    Foo,
    Bar(T),
}

let store = use_store(|| Enum::Bar("Hello".to_string()));

let transposed = store.transpose();
use Enum::*;
match transposed {
    Enum::Foo => {}
    Bar(bar) => {
        let bar: String = bar();
    }
}