[#1511] Refactor account representation from int to a dedicated account structure

[LukasKorba]

Closes #1511

• DerivationTool uses accountIndex: Int32
• from_account_id/to_account_id refactored to AccountId(Int)
• the rest has been refactored to Zip32Account(Int32)

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[daira]

Changes requested.

[daira]

utACK with blocking comments.