prog: mutate programs more aggressively
prog: mutate programs more aggressively

Currently we stop mutating with 50% probability.
Stop mutating with 33% probability instead.
Benchmark shows both coverage increase and corpus reduction:

                    baseline          oneof3            diff
coverage               65467           65604             137
corpus                 35423           35354             -69
exec total           5474879         5023268         -451611

google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,dotweiba/syzkaller,zoulasc/syzkaller,hyz0906/syzkaller,xairy/syzkaller,google/syzkaller,dotweiba/syzkaller,zoulasc/syzkaller,google/syzkaller,zoulasc/syzkaller,google/syzkaller,hyz0906/syzkaller,xairy/syzkaller,google/syzkaller,hyz0906/syzkaller,xairy/syzkaller,zoulasc/syzkaller,hyz0906/syzkaller,xairy/syzkaller,dotweiba/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller