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/rpcdisclosure.cpp

@@ -254,11 +254,8 @@ UniValue z_validatepaymentdisclosure(const UniValue& params, bool fHelp)
    
     // Check the payment address is valid
     PaymentAddress zaddr = pd.payload.zaddr;
-    CZCPaymentAddress address;
-    if (!address.Set(zaddr)) {
-        errs.push_back("Payment disclosure refers to an invalid payment address");        
-    } else {
-        o.push_back(Pair("paymentAddress", address.ToString()));
+    {
+        o.push_back(Pair("paymentAddress", EncodePaymentAddress(zaddr)));
 
         try {
             // Decrypt the note to get value and memo field