Create new trait for non-dedup storage decode (#1932)

- This adds the new trait `StorageDecodeNonDedupLength` and implements
them for `BTreeSet` and its bounded types.
- New unit test has been added to cover the case.  
- See linked
[issue](https://github.com/paritytech/polkadot-sdk/issues/126) which
outlines the original issue.

Note that the added trait here doesn't add new logic but improves
semantics.

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Co-authored-by: joe petrowski <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: command-bot <>