Add changed keys wrapper for WlStorage

[sug0]

Introduce a WlStorage wrapper that keeps track of changed storage keys, due to writes or removals. The wrapper should have a finalize() method, that returns a BTreeSet<Key> of changed storage keys. It is not uncommon for me (personally) to forget adding storage key changes to a set of changed keys, so this abstraction would take this burden away by keeping track of the changed keys internally.

[tzemanovic]

@sug0 I'm not sure if I follow, doesn't the write log already contain this?

[sug0]

@tzemanovic guess so. the motivation for this was that in the ethereum bridge code we were manually updating a BTreeSet of changed keys, and would forget keys sometimes. the bigger issue here is a need for a refactor on the eth bridge code :D