Maybe bounds for recursive types can be improved

This is a potential improvement but it doesn't completely solve the issue for recursive types so not sure it matters, I'm OK to close.

the following example doesn't compiles:

#[test]
fn recursive_variant_3_encode_works() {
	#[derive(Default)]
	struct NonEncodableType;

	#[derive(Default, Encode)]
	struct Recursive<A, B, N> {
		other: Vec<(A, B, Recursive<A, B, N>)>,
		n: core::marker::PhantomData<N>,
	}

	let val: Recursive<u32, i32, NonEncodableType> = Recursive::default();
	val.encode();
}

this is because the bounds are:

A: Encode,
B: Encode,
N: Encode,
core::marker::PhantomData<N>: Encode,

for recursive types like Vec<(A, B, Recursive<A, B, N>)>, I think we could only bound all used generics but not those inside Self types: thus A and B here.

(I tested and bounding only A, B, and phantom make the test works, though I haven't tried substrate codebase)

Though anyway this is just best effort, ideally we should bound Vec<(A, B, Recursive<A, B, N>)>: Encode but this is not possible.

Start by reproducing the recursive_variant_3_encode_works example from the issue and inspect the derive-generated bounds for Recursive. Determine whether recursive field types can avoid requiring Encode for N; done means the example compiles while retaining correct bounds for A, B, and the relevant PhantomData type.