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/base58.cpp

@@ -318,3 +318,25 @@ bool CBitcoinSecret::SetString(const std::string& strSecret)
 {
     return SetString(strSecret.c_str());
 }
+
+std::string EncodeDestination(const CTxDestination& dest)
+{
+    CBitcoinAddress addr(dest);
+    if (!addr.IsValid()) return "";
+    return addr.ToString();
+}
+
+CTxDestination DecodeDestination(const std::string& str)
+{
+    return CBitcoinAddress(str).Get();
+}
+
+bool IsValidDestinationString(const std::string& str, const CChainParams& params)
+{
+    return CBitcoinAddress(str).IsValid(params);
+}
+
+bool IsValidDestinationString(const std::string& str)
+{
+    return CBitcoinAddress(str).IsValid();
+}