UnorderedMap: make it accept Devices or MemorySpaces

[crtrott]

It is useful to tell an unordered map where to store its memory independently of the execution space. For example on KNL I might need to decide whether to put the map in HBM or DDR memory. This can be done by making the map accept Kokkos::Device instead of an execution space as the views allow.

This will automatically make it work for execution and memory spaces since both are devices as well.

I already made that change and pushed it.