After we've detected that we need to change the container in insert(), don't detach(). Instead, check if we're shared. If we're not shared, do the change in-place. Otherwise, make a new container, copy the subset that forms the final result over, then assign the new container to *this.

See epic for more details.

Acceptance criteria: The following member functions no longer detach:

• emplace*() (all variants)
• insert() (all overloads)