Allow user to suppress result cohorts for uniform negatives

[woodruffw]

Right now, mishegos spits out every single cohort for every single candidate tried, even if every decoder agrees that the input is invalid. This results in extremely large outputs by default, since even guided searches of the x86 encoding space produce large numbers of garbage instruction candidates.

We should preserve the default behavior, but introduce a new flag/environment variable that tells mishegos to filter out uniformly negative results.