Refactor of `ivf_flat::index` to split the cluster data in separate buffers
- Addressing rapidsai#1170.
- Following rapidsai#1249 in the index structure implementation.
- Adding serialization API with stream and filename overloads
- Moving `raft::spatial::knn::ivf_flat` namespace to `raft::neighbors::ivf_flat`

Authors:
  - Micka (https://github.com/lowener)
  - Corey J. Nolet (https://github.com/cjnolet)
  - Artem M. Chirkin (https://github.com/achirkin)

Approvers:
  - Tamas Bela Feher (https://github.com/tfeher)
  - AJ Schmidt (https://github.com/ajschmidt8)
  - Artem M. Chirkin (https://github.com/achirkin)

URL: rapidsai#1271