Ensure the caller is required to provide the diversifier index for address derivation

[pacu]

Suggestion from @nuttycom
#491 (comment)

public func deriveUnifiedAddress(seed: [UInt8], accountIndex: Int) throws -> UnifiedAddress

change API so that the caller is required to provide the diversifier index; with unified addresses, we definitely want to avoid address reuse where possible.

[pacu]

This has been dropped to favor get_current_address and get_next_address APIs for rolling diversified addresses

cc @nuttycom