storage keys parsed type doesn't preserve order

[tzemanovic]

The storage::Key type should have the same order as in its raw string/bytes form. Specifically, this is important for prefix iterators that in the DB follow the order of raw bytes, but in other places like WriteLog where we use storage::Key, the order doesn't match. In #714 we found that this occurs when the storage keys contain addresses, but we should ensure consistency for all the possible values.

Related to #30, I think in long term, the storage::Key type would better be represented as raw bytes instead of the parsed key segments in memory and we can apply segments parsing on demand, which besides other advantages would prevent this issue altogether.