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/guiutil.cpp

@@ -112,8 +112,9 @@ static std::string DummyAddress(const CChainParams &params)
     sourcedata.insert(sourcedata.end(), dummydata, dummydata + sizeof(dummydata));
     for(int i=0; i<256; ++i) { // Try every trailing byte
         std::string s = EncodeBase58(sourcedata.data(), sourcedata.data() + sourcedata.size());
-        if (!CBitcoinAddress(s).IsValid())
+        if (!IsValidDestinationString(s)) {
             return s;
+        }
         sourcedata[sourcedata.size()-1] += 1;
     }
     return "";
@@ -248,7 +249,7 @@ QString formatBitcoinURI(const SendCoinsRecipient &info)
 
 bool isDust(const QString& address, const CAmount& amount)
 {
-    CTxDestination dest = CBitcoinAddress(address.toStdString()).Get();
+    CTxDestination dest = DecodeDestination(address.toStdString());
     CScript script = GetScriptForDestination(dest);
     CTxOut txOut(amount, script);
     return IsDust(txOut, ::dustRelayFee);