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/script/standard.cpp

@@ -316,3 +316,7 @@ CScript GetScriptForMultisig(int nRequired, const std::vector<CPubKey>& keys)
     script << CScript::EncodeOP_N(keys.size()) << OP_CHECKMULTISIG;
     return script;
 }
+
+bool IsValidDestination(const CTxDestination& dest) {
+    return dest.which() != 0;
+}