Avoid wasted lookup in insert_or_assign method

[mohitmv]

Comment from reddit

Implementation of at would be more efficient if you got rid of the first call to contains_key and simply used find directly. You call contains_key to check if something is in the map (doing a map lookup in the process) then, if so, immediately call at on the underlying map (doing a redundant map lookup) to access the value.

insert_or_assign, move, and move_only all have the same problem. move_and_erase is even worse - you either call at on yourself or operator[] on the underlying map, so you do a wasted lookup in BOTH branches.

https://www.reddit.com/r/cpp/comments/m9uwaq/releasing_lazy_map_c_library/grpmxlp?utm_source=share&utm_medium=web2x&context=3