Add an overload of insert_check taking a key_type

[fdegros@…]

The different flavors of intrusive set don't seem to have an insert_check method that takes a key_type directly and uses the container's key comparator and/or hasher.

It would be convenient to add this method:

std::pair<iterator, bool> 
insert_check(const key_type&, insert_commit_data&);

That would also be consistent with the way the methods find, lower_bound, upper_bound, equal_range and erase work.

[Ion Gaztañaga]

Thanks for the report. You are right they were missing. Fixed in:

​https://github.com/boostorg/intrusive/commit/4cf38987a88e1bb1157f1b8957bc0fc167b576e4