Introduce wrappers around CBitcoinAddress

This patch removes the need for the intermediary Base58 type
CBitcoinAddress, by providing {Encode,Decode,IsValid}Destination
function that directly operate on the conversion between strings
and CTxDestination.

src/qt/bitcoinaddressvalidator.cpp

@@ -89,9 +89,9 @@ QValidator::State BitcoinAddressCheckValidator::validate(QString &input, int &po
 {
     Q_UNUSED(pos);
     // Validate the passed Bitcoin address
-    CBitcoinAddress addr(input.toStdString());
-    if (addr.IsValid())
+    if (IsValidDestinationString(input.toStdString())) {
         return QValidator::Acceptable;
+    }
 
     return QValidator::Invalid;
 }