Add overload to set_difference_by_key(...) that doesn't require values2

[aabbas90]

First of all, great library!

I am having some confusion in understanding the role of values_first2 in set_difference_by_key as mentioned here.

In general terms, the result of set difference D = A - B will not contain values from B therefore values_first2 can never be part of D. So values_first2 will never be used in this function and thus should be removed as an input argument. Is my understanding correct? Thanks.

[alliepiper]

This is a good observation. You are correct, the values for the second set are not strictly necessary to compute the set difference.

However, all of the set operations share a common generic implementation that is parameterized for the different operations. This generic implementation expects both ranges to define both keys and values.

If you'd like to save some memory and/or transfer expense, you can pass in a storageless iterator (like thrust::constant_iterator) as values2, since they shouldn't be used.

At some point we may add a convenience overload that doesn't require values2 in its API and inserts a storageless iterator automatically, but this will be low priority since there's an easy workaround.