Add overload to set_difference_by_key

[eriktedhamre]

Description

Hello!
Unsure if this is a correct interpretation and solution to the issue below.
I would appreciate feedback!

closes [https://github.com//issues/785]

Added overload for set_difference_by_key(....) that uses thrust::constant_iterator for values_first2.
I'm not sure if this should be extended to also exist for other version, for example the one with a custom comparator.

Checklist

I tried to add a simple test case to test my change, not user if this is the correct way to go about it. The documentation for how tests should be written for thrust is not as extensive as the one for CUB.
In regards to documentation, I have not changed anything. Because I'm unsure about what to do.

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[eriktedhamre]

That merge should probably have been a rebase.

[jrhemstad]

Thanks @eriktedhamre, we'll take a look at this right away!

That merge should probably have been a rebase.

No worries, we're not picky about merge commits in PRs because we just squash merge them 🙂