Maybe we should implement the Matlab behavior in assert and use
test_assert or tassert, or something else for the Octave assert
functionality that we use in the test suite? Then we could easily
make assert a built-in function and it would be somewhat faster too,
solving the initial performance concern.
We could leave the !%assert syntax alone, but have it call the new
test_assert function.
The built-in assert function could test for the Matlab style case and
perform that action, otherwise warn about deprecated usage and call
the new test_assert function.