Implement wrapping of EPM types (#1633)

This PR wraps the `Snapshot`, `SnapshotMetadata` and `DesiredTargets`
storage items in the [EPM
pallet](https://paritytech.github.io/substrate/master/pallet_election_provider_multi_phase/index.html)
in order to keep them in sync throughout the election lifetime and in
order to be killed together.

Prior to this PR, these storage items were mutated in different places
in the pallet;

In addition 2 helper `fns` are introduced for chekcing if all the
wrapped storage items exist or not;

Fixes #413 ;

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Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>