Auto merge of #3579 - Eirik0:3577-generate-sprout-zkey, r=bitcartel

Make it clear that CWallet::GenerateNewZKey is Sprout specific

Closes https://github.com/zcash/zcash/issues/3577

When adding sapling support we had considered making the method `GenerateNewZKey` generic, but it ended up making more sense to add a second method `GenerateNewSaplingZKey` for sapling support.

This PR changes the name of `GenerateNewZKey` to `GenerateNewSproutZKey` and changes the return type from `libzcash::PaymentAddress` to `libzcash::SproutPaymentAddress` to make it more clear that this is what the method does.

src/wallet/wallet.cpp

@@ -82,23 +82,23 @@ const CWalletTx* CWallet::GetWalletTx(const uint256& hash) const
 }
 
 // Generate a new spending key and return its public payment address
-libzcash::PaymentAddress CWallet::GenerateNewZKey()
+libzcash::SproutPaymentAddress CWallet::GenerateNewSproutZKey()
 {
     AssertLockHeld(cs_wallet); // mapSproutZKeyMetadata
-    // TODO: Add Sapling support
+
     auto k = SproutSpendingKey::random();
     auto addr = k.address();
 
     // Check for collision, even though it is unlikely to ever occur
     if (CCryptoKeyStore::HaveSproutSpendingKey(addr))
-        throw std::runtime_error("CWallet::GenerateNewZKey(): Collision detected");
+        throw std::runtime_error("CWallet::GenerateNewSproutZKey(): Collision detected");
 
     // Create new metadata
     int64_t nCreationTime = GetTime();
     mapSproutZKeyMetadata[addr] = CKeyMetadata(nCreationTime);
 
     if (!AddSproutZKey(k))
-        throw std::runtime_error("CWallet::GenerateNewZKey(): AddSproutZKey failed");
+        throw std::runtime_error("CWallet::GenerateNewSproutZKey(): AddSproutZKey failed");
     return addr;
 }