Add whenMissing, which takes user-supplied fns

[meooow25]

This allows for some useful applications which are currently not possible with the non-internal API. The functions carry a warning that the two input functions are related by a law and the user is responsible for satisfying it.

Export filterA for Set, now that it is referred to from whenMissing.

Closes #1054

[meooow25]

I decided to go with the simple approach from #1054.

Just a note that I haven't added this function to the Merge.Strict modules, since I cannot think of a situation where we would also need to force the values.

[treeowl]

This makes me pretty nervous. One of the major reasons I wrote the new merge interface was to get away from the earlier approach with pretty complex preconditions on user-passed arguments.

[meooow25]

The motivation here is that it is actually useful to allow this, e.g. to calculate union and intersection at once (#944). An analogy might be lens allowing you define your own Lens, provided you satisfy the laws.

The alternative would be to tell people that they need to use the internal module to do what they want to do. The functionality is less discoverable and has less stability guarantees.

[treeowl]

The representation of WhenMissing also has no stability guarantees. In fact, my original implementation sketch made it a GADT! That said, making such a guarantee probably wouldn't hurt too much. Let me take a look at your example uses. At the very least, I think the preconditions need to take on a sterner tone.

[meooow25]

@treeowl did you get a chance to look at the examples?