Test consistently that the strings passed in input_arrays and output_arrays

consist of printable ASCII characters (this is motivated by a user having
unwittingly passed unicode zero-width characters, probably by copy-pasting),
and are names of arrays actually existing in the model.
Centralize these tests in CheckInvariants.

This can be overridden with new model flags: --allow_nonascii_arrays,
--allow_nonexistent_arrays. These are model flags because this is about
self-consistency of the model and its existing modelflags.

This CL partly undoes a recent relaxation of checks on input arrays that
was done to support getting graphviz out of incorrectly specified graphs.
Such users will now have to pass --allow_nonexistent_arrays.

PiperOrigin-RevId: 178939235