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

In order to avoid unintended implicit conversions.

src/test/dbwrapper_tests.cpp

@@ -231,7 +231,7 @@ struct StringContentsSerializer {
     // This is a terrible idea
     std::string str;
     StringContentsSerializer() {}
-    StringContentsSerializer(const std::string& inp) : str(inp) {}
+    explicit StringContentsSerializer(const std::string& inp) : str(inp) {}
 
     StringContentsSerializer& operator+=(const std::string& s) {
         str += s;