Inconsistency in cudf directory and file structures

Currently, there has been a number of inconsistency issues in `cudf` directory and file structures. Below are some examples:

* The current directory structures of `cpp/include`, `cpp/src`, `cpp/tests` and `cpp/benchmarks` are inconsistent. For instance, there are tests in `tests/grouped_rolling` which test `grouped_rolling_window` corresponding to `src/rolling`; or unit tests in `tests/collect_list` and `tests/lead_lag` that are all tests for `cpp/aggregation`.
* There are `tests/utilities` and `tests/utilities_test` which have confusing names.
* There are tests in `tests/column/colum_view_test.cpp` which are actually testing `logical_cast` (or `bit_cast`), so they should be put together with the tests in `tests/unary/cast_tests.cpp`.
* File name/extensions are inconsistent. For example, many unit tests in the `tests/` are written in both `.cu` and `.cpp` extensions. Of course, `.cu` file may be required here, but having the tests written in two file extensions at the same time make things look messy.

And many other similar issues that are not mentioned here. I would recommend to refactor and enforce more consistency.