Array function to filter for unique elements

[msoeken]

This implements Unique from the proposal in #313.

Note that this function implements Unique wrt. the semantics, e.g., found in C++.

It returns an array with the same order but no adjacent equal elements. If one wants to have truly unique arrays, one can combine this function with Sorted (mentioned in the docs).

If we want a different semantics, we might need to add a comparison function to check for order, or a more general comparison function that returns (-1 for <, 0 for =, and 1 for >).

[cgranade]

Looks good, thanks for adding this!

[guenp]

Looks like a good and useful addition! Just would suggest different naming as I'm not sure most people are familiar with the C++ semantics. And, would it make sense to actually add a TrulyUnique function as well, that returns all unique elements in an array?

[guenp]

After some discussion, it looks like this was already discussed in the API design meeting, so I approve this PR.