Fix & test pretty-printing 0-dimensional region maps

[fknorr]

region_map_impl is specialized on the 0-dimensional case but the GDB pretty printers weren't, causing them to try and access non-existing members. This PR adds the necessary pretty-printer code and adds a test.

[github-actions[bot]]

Check-perf-impact results: (4c65f1399a47e0eb1340f63004745b17)

:question: No new benchmark data submitted. :question:
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[coveralls]

Pull Request Test Coverage Report for Build 7887220924

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 93.906%

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Files with Coverage Reduction	New Missed Lines	%
src/grid.cc	1	97.29%
<!--	Total:	1		-->

Totals
Change from base Build 7875546105:	-0.04%
Covered Lines:	4998
Relevant Lines:	5194

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💛 - Coveralls