provide machine-readable, reliable output for result of checking realizability etc.

[slivingston]

Currently gr1c uses exit codes both to indicate errors (including unhandled signals, e.g., segmentation faults) and to report some results, e.g., realizability. This can cause confusion when other programs are interpreting exit codes, especially in situations where negative exit codes are converted to positive integers.

To address this, gr1c should give output in a machine-readable form for all operations (not only synthesis) and outcomes. Thus, other programs would only need to consider nonzero exit codes as errors and can otherwise only attempt to parse gr1c output. Consider keeping some or all of the current usage of exit codes from gr1c to facilitate shell scripting.

This feature request is motivated by a comment in tulip-control/tulip-control#152.