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

In order to avoid unintended implicit conversions.

src/script/standard.cpp

@@ -253,7 +253,7 @@ class CScriptVisitor : public boost::static_visitor<bool>
 private:
     CScript *script;
 public:
-    CScriptVisitor(CScript *scriptin) { script = scriptin; }
+    explicit CScriptVisitor(CScript *scriptin) { script = scriptin; }
 
     bool operator()(const CNoDestination &dest) const {
         script->clear();