Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/base58.cpp

@@ -218,7 +218,7 @@ private:
     CBitcoinAddress* addr;
 
 public:
-    CBitcoinAddressVisitor(CBitcoinAddress* addrIn) : addr(addrIn) {}
+    explicit CBitcoinAddressVisitor(CBitcoinAddress* addrIn) : addr(addrIn) {}
 
     bool operator()(const CKeyID& id) const { return addr->Set(id); }
     bool operator()(const CScriptID& id) const { return addr->Set(id); }