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ARROW-16590: [C++] Consolidate files dealing with row-major storage (#13218)

The primary goal of this refactor of old code was to improve the readability and clarity of the code base. I did not make any functional changes to the code and if any functional changes are suggested which modify existing code I will happily discuss them here but defer the changes themselves to follow-up PRs. I would very much appreciate any feedback on naming, making sure we have sufficient test coverage, and overall layout of the code.

* KeyRowArray -> RowTableImpl KeyEncoder -> RowTableEncoder: The old name made sense because this data is currently represented physically as an array of rows. However, the data is conceptually tabular. We are storing rows & columns. In particular, I found it confusing that `KeyColumnArray` was a 1D data structure while `KeyRowArray` was a 2D table structure.
* KeyEncoder::Context -> LightContext: There's nothing particular to the key encoder here and I worry keeping it there may lead to fracturing into many different "context" objects. 
* Overall structure: I created a new folder arrow/compute/row and put all row-based utilities in here. Most of the files are now marked as _internal and the content in these files is not used outside of arrow/compute/row. The grouper had previously been alongside the kernel code and it didn't really belong there as it relies very heavily on the internal structure of the row encoding.
* Row structure: I documented the file arrow/compute/row/row_internal.h

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>