All implicitly-shared containers: avoid detaching when the size would change (esp. remove)

In almost all remove-like operations, our containers first detach and then do the remove. This is inefficient, because we spend time copying elements we may not want in the final result.

If *this is shared, then we should not detach. Instead, we should make a new container, and copy only those that would have been kept.

iow, in pseudo-STL:

- auto copy = c;
- remove(copy, x);
+ decltype(c) copy;
+ remove_copy(c, copy, x);

A similar argument can be made for operations that grow the container, like insert().