Migrate to Rust backend with fast spendability support

[str4d]

Depends on Electric-Coin-Company/zcash-light-client-ffi#94.

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[LukasKorba]

So far the code looks good, I only noticed we should remove the ZcashErrors related to Validation as that one has been removed. I'll go through errors first so I'm sure we remove the right ones.

Anyway, when testing the functionality I've got stuck at the beginning, the Synchronizer calls prepare() and there

        if case .seedRequired = try await self.initializer.initialize(
           with: seed, 
           viewingKeys: viewingKeys, 
           walletBirthday: walletBirthday) {

is called.. eventually ZcashRustBackend's rustBackend.initDataDb(seed: seed) calls zcashlc_init_data_database and that one seems to start some sort of migration or tree/subtree magic but loops over and over.

[nuttycom]

post-hoc utACK