[FRAME] Storage derive macro #[frame_support::stored]

[ggwpez]

WIP DNM

Some examples:

/// Ignore T and require S to be MEL.
#[frame_support::stored(skip(T), mel(S))]
struct A<S, T>(::core::marker::PhantomData<(S, T)>);

/// Only create a custom bound for MEL:
#[frame_support::stored(skip(S, T), mel_bound(S: MaxEncodedLen + Encode, T: MaxEncodedLen + Encode))]
struct A<S, T>(::core::marker::PhantomData<(S, T)>);

/// Overwrite the standard MEL bound with a custom one.
#[frame_support::stored(mel_bound(S: MaxEncodedLen + Encode, T: MaxEncodedLen + Encode))]
struct A<S, T>(::core::marker::PhantomData<(S, T)>);

[ggwpez]

The difficult part about this was to re-add the Clone etc. bounds for all parameters that are not skipped.
And that requires some changes to all of our *NoBound derives. Writing all these bounds like in this case is not so nice... ideally it would do it automatically.

[muharem]

if I read it right, mel(Votes) here means do not generate anything for Votes and use whatever is set for it below? wonder why MaxEncodedLen is not needed then?

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6116337