[#824] Refactor aliases storage in Cleveland

Description

Problem: We're storing aliases in three five places:

• PureState._psAliases: stores aliases of contracts and implicit accounts (and secret keys of implicit accounts).
• PureState._psContractsNames: stores aliases of contracts.
• GState.gsAddressAliases: stores aliases of contracts.
• PureState._psImplicitAliases: stores aliases (and secret keys) of implicit accounts.
• PureState._psContractAliases: stores aliases of contracts.
• PureState._psContractsNames: stores aliases of contracts.
• GState.gsImplicitAddressAliases: not currently used, it's always empty.
• GState.gsContractAddressAliases: stores aliases of contracts.

This MR:

• Stores aliases of implicit accounts in GState.gsImplicitAddressAliases. GState is now the single source of truth for aliases.
• Deletes PureState._psContractsNames and _psContractAliases as they're entirely redundant.
• Delete alias data from PureState._psImplicitAliases, replace it with _psSecretKeys :: Map Address SecretKey.~

It also deletes GSAddImplicitAddressAlias, as it's not being used anywhere.

See each commit's message/contents - each of the above steps is done in an individual commit to make it easier to review.

Related issue(s)

Resolves #824 (closed)

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