Add dedup_len trait

[0xmovses]

This is a fix for paritytech/polkadot-sdk#126

[bkchr]

Please revert the FMT commit.

[ggwpez]

Wait... this adds a DecodeDedupLength trait for BTreeSet? I dont think thats possible. The advantage of the DecodeLen is that is does not have to decode everything, but only a Compact<u32>.
What i meant in the other issue to just remove the impl DecodeLen for BTreeSet and replace it with a DecodeNonDedupLen or so.
cc @bkchr

[0xmovses]

Yeah that's clearer now, let me update this.

[0xmovses]

I don't think we need to add this here anymore, as we are just adding a more descriptive trait in polkadot-sdk, closing this.