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

@@ -317,3 +317,7 @@ CScript GetScriptForWitness(const CScript& redeemscript)
     ret << OP_0 << ToByteVector(hash);
     return ret;
 }
+
+bool IsValidDestination(const CTxDestination& dest) {
+    return dest.which() != 0;
+}

src/script/standard.h

@@ -77,10 +77,13 @@ public:
  *  * CNoDestination: no destination set
  *  * CKeyID: TX_PUBKEYHASH destination
  *  * CScriptID: TX_SCRIPTHASH destination
- *  A CTxDestination is the internal data type encoded in a CBitcoinAddress
+ *  A CTxDestination is the internal data type encoded in a bitcoin address
  */
 typedef boost::variant<CNoDestination, CKeyID, CScriptID> CTxDestination;
 
+/** Check whether a CTxDestination is a CNoDestination. */
+bool IsValidDestination(const CTxDestination& dest);
+
 /** Get the name of a txnouttype as a C string, or nullptr if unknown. */
 const char* GetTxnOutputType(txnouttype t);