Introduce wrappers around CZCPaymentAddress

This patch removes the need for the intermediary Base58 type
CZCPaymentAddress, by providing {Encode,Decode}PaymentAddress
functions that directly operate on the conversion between strings
and libzcash::PaymentAddress.

src/wallet/wallet.h

@@ -960,7 +960,7 @@ public:
       * ZKeys
       */
     //! Generates a new zaddr
-    CZCPaymentAddress GenerateNewZKey();
+    libzcash::PaymentAddress GenerateNewZKey();
     //! Adds spending key to the store, and saves it to disk
     bool AddZKey(const libzcash::SpendingKey &key);
     //! Adds spending key to the store, without saving it to disk (used by LoadWallet)