Efficiency of implicit inserts

[Mogball]

Currently implicit inserts in json_array and json_object wrap the inserted type, whether json_element, dynamic_string or some primitive type in another json_element as

insert(json_element(forward<val_t>(val)));

Firstly, this is redundant when inserting json_element but I also suspect that, for other types, this introduces one unnecessary move/copy operation, that is, (assuming val_t is not `json_element)

1. The value is moved/copied into a json_element rvalue instance
2. That json_element is then moved into pair instance
3. Upon insertion, the said json_element is moved into the container

If we could open up the container machinery we could cut out the middle step and move/copy the value directly into the container.

This applies for object insert, insert_or_assign, and array push_back, push_front, insert etc.