Key Dependent Signer Function APIs

[SupremoUGH]

Goes together with Manta-Network/sdk#85

Changes for Signer and SignerState in manta-accounting::wallet::signer:

• SignerState, instead of an AccountTable to be initialized, now it has Option<AccountTable> and Option<AuthorizationContext>.
• sign requires AccountTable to be there. New field in SignError if it's None.
• sync requires AuthorizationContext to be there. New field in SyncError if it's None.
• New methods: instantiation without AccountTable, and loading/dropping AccountTable and AuthorizationContext.

More changes with design decisions:

• address requires AccountTable. We could require the weaker AuthorizationContext for this, or, have an Address field in SignerState because it is the lowest level of security.
• address now returns Option<Address> because of this.
• transaction_data requires AccountTable. We could require the weaker AuthorizationContext for this or, have a DecryptionKey field in SignerState because that's all it's needed. However, it makes sense to require the AccountTable because it's commonly used in the compound method sign_with_transaction_data, which does require AccountTable.
• transaction_data and identity_proof still return Option. Perhaps it makes sense that they return more verbose Results now?

manta-pay::signer::functions.rs changes:

• Function new_signer initializes the signer with just parameters and optionally the state.
• Derivation of all keys from a Mnemonic.

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