Use a move asssignment instead of a copy assignment when returning a result

via detail::make_parse_result().

Supercedes PR #247.

include/boost/parser/parser.hpp

@@ -1548,7 +1548,7 @@ namespace boost { namespace parser {
         {
             std::optional<T> retval;
             if (success)
-                retval = x;
+                retval = std::move(x);
             return retval;
         }