limex: implement variable shift NFA engines

Replaces the old LimEx NFA engines, which were specialised for model
size and number of shifts, with a new set of engines that can handle a
variable number of shifts.

src/grey.h

@@ -88,7 +88,6 @@ struct Grey {
     bool floodAsPuffette;
 
     u32 nfaForceSize;
-    u32 nfaForceShifts;
 
     u32 maxHistoryAvailable;
     u32 minHistoryAvailable;