Document and export `Base.in!` (#51636)
I think `in!` is a useful general function for users, and would be good
to have as official API. Its semantics is clear and unambiguous, while
providing a clear performance advantage over the naive implementation.
For more evidence that this functionality is useful, consider:
* Rust's `HashSet::insert` works just like this implementation of `in!`
* This function was already used in the implementation of `Base.unique`,
precisely for the performance over the naive approach
Comes from #45156 with some initial discussion.
