Add safe_print() support for Kind enum (#4213)

Add safe_print() support for Kind enum (#4213)

This commit changes the mkkind script to generate a toString()
method for the Kind enum. This method can be used by the
safe_print() function to print statistics if CVC4 has been terminated
before solving a problem. The stats for strings include statistics that
rely on printing kinds (e.g. the number of reductions done of each
kind).