[REVIEW] Change strings::split_record to return a lists column

[davidwendt]

Reference #5667

The cudf::strings::contiguous_split_record API returns a single memory buffer plus a vector of column_views similar to the cudf::contiguous_split function. Now that we have LIST type columns, the strings API has been repurposed in this PR to return a lists column instead. The lists column child will be a flat strings column and the list offsets identify each input string's tokens.

Since the new APIs no longer matches the contiguous_split result, this PR also changes the names to simply cudf::strings::split_record and cudf::strings::rsplit_record. All appropriate gtests have been updated accordingly.

The code change was involved because the original layout of characters and offsets were interleaved in the output memory. The new result creates one large strings column of all the tokens along with the appropriate offsets for the lists column.

This API is not currently exposed in the cudf Python interface but could be used in the str.split() functions with expand=False

This PR does not include adding regex support to any of the existing split() APIs.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov]

Codecov Report

Merging #5687 into branch-0.15 will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           branch-0.15    #5687   +/-   ##
============================================
  Coverage        86.38%   86.38%           
============================================
  Files               76       76           
  Lines            13041    13041           
============================================
  Hits             11265    11265           
  Misses            1776     1776           

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[harrism]

Does this close #5667, or just "reference" it?

[davidwendt]

Does this close #5667, or just "reference" it?

This PR includes only the libcudf C++ code for this API and not any of the Python/Cython bindings to it.
The Lists support for Python cudf is ongoing so the reference issue will likely be closed in a future PR.

[revans2]

Just one nit that I saw. Excited to start using this when it is merged in.

[nvdbaranec]

Looks good.

[revans2]

Looks good to me too.