state crate refactor

[tzemanovic]

• unify names in the namada_state as its usages (struct State was previously Storage, but we still have e.g. WlStorage that wraps the State, WriteLogAndStorage [and its methods], TempWlStorage, inside Shell, etc.)
• make the State internal to the crate, from the outside it should only be accessed via what's currently called WlStorage - again, the names are confusing)