library/version_array2d.m:
For every predicate or function that takes an integer argument,
if that integer is required to be non-negative, add an unsigned version.
Add bounds checks to lookup (read) and set (write) operations.
Improve the documentation of the exported operations.
Add a new implementation of the function that returns the rows
of the array as a list of lists.
Improve variable names in operations that have new unsigned versions.
NEWS.md:
Announce the new operations.
tests/hard_coded/version_array2d_test.{m,exp}:
New test case to test the operation of version_array2d.m.
It is based on the applicable parts of version_array_test.m,
but with somewhat improved organization.
tests/hard_coded/Mmakefile:
Enable the new test case.
tests/hard_coded/version_array_test.m:
Minor style fixes.
