[FEA] Move view classes to their own header

[sleeepyjack]

Part of #110 (Refactor of open address data structures)

Development branch: NVIDIA/cuCollections/refactor

Synopsis

Device view classes are currently implemented as nested classes of their associated data structures. This results in

TODOs

• Regroup header files by data structure, e.g. include/cuco/detail/static_map/{kernels.cuh, impl.inl, views.cuh, ...}
• Move views to their own header files
• To disambiguate view types among data structures, use inline namespaces as shown here: https://godbolt.org/z/4qr8MK89o
• Find a way to reduce code duplication when redefining member types/passing down tparams. Use case for CRTP, e.g., device_view<MapType>?

References

• https://godbolt.org/z/4qr8MK89o

[PointKernel]

completed