sby_design: Also track fairness assumptions

[jix]

If we track $assume we should also track $fair and certainly need to support $check cells where the flavor is fair.

[nakengelhardt]

Fairness properties aren't supported at all yet in sby, are they?

[jix]

$fair is roughly to $live as $assume is to $assert so $fair would be used for for liveness properties and in fact the one liveness example included in SBY does have a $fair cell which sby_design so far ignored. With a $check cell it wouldn't ignore it but instead error out making the test for that example fail.