fix scale support for upcoming versions

[pgherveou]

This PR should fix breaking changes for upcoming parity-scale-codec versions

Motivation

We have added some constraints to the HasCompact trait in paritytech/parity-scale-codec#512
to fix MaxEncodedLen for compact fields.

/// Trait that tells you if a given type can be encoded/decoded in a compact way.
pub trait HasCompact: Sized {
	/// The compact type; this can be
-	type Type: for<'a> EncodeAsRef<'a, Self> + Decode + From<Self> + Into<Self>;
+	type Type: for<'a> EncodeAsRef<'a, Self> + Decode + From<Self> + Into<Self> + MaybeMaxEncodedLen;
}

This is so that we can fix the MaxEncodedLen derive trait for Compact field

- ty.span() => .saturating_add(<#crate_path::Compact::<#ty> as #crate_path::MaxEncodedLen>::max_encoded_len())
+ ty.span() => .saturating_add(<<#ty as #crate_path::HasCompact>::Type as #crate_path::MaxEncodedLen>::max_encoded_len())

This change breaks uint, thus this PR to attempt to fix it.

Solution

This PR implements MaxEncodedLen for CompactUint (and Encode as required) to fix the compilation issue with upcoming versions

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[pgherveou]

I need to implement this as MaxEncodedLen inherits from Encode, we can't get away without it

[pgherveou]

🤔 these max_encoded_len values seems to be off,
I would have expected this test to pass

    #[test]
    fn test_scale_compact_max_len() {
        const_for!(BITS in [1, 2, 3, 7, 8, 9, 15, 16, 17, 29, 30, 31, 32, 33, 63, 64, 65, 127, 128, 129, 256, 384, 512, 535] {
            const LIMBS: usize = nlimbs(BITS);
            let value: CompactUint<BITS, LIMBS> = Uint::<BITS, LIMBS>::MAX.into();
            assert!(CompactUint::<BITS, LIMBS>::max_encoded_len(), value.encode().len());
        });
    }

[prestwich]

Closing as stale. We will support this once 4.0.0 comes out :)

more context here:
paritytech/parity-scale-codec#662