Update constructor of jsont based on the copy and move idiom.

This improves the case where `jsont` is constructed based on
the result of a function. In this case the copy will be elided and
`value` will be initialised based on the move constructor.

Author: thomasspriggs

src/util/json.h

@@ -117,7 +117,7 @@ class jsont
   {
   }
 
-  jsont(kindt _kind, const std::string &_value):kind(_kind), value(_value)
+  jsont(kindt _kind, std::string _value):kind(_kind), value(std::move(_value))
   {
   }