Use Integer in towards for halving during shrinking (#397)

* Use Integer in towards for halving during shrinking

This fixes the odd situation in which 2 is not representable as a number. In
particular this occurs in the package 'clash-prelude', which defines types
such as `Unsigned 1` where only 0 and 1 can be represented.

* Special case 1-bit numbers

Co-authored-by: Jacob Stanley <jacob@stanley.io>

hedgehog/src/Hedgehog/Internal/Shrink.hs

@@ -27,6 +27,9 @@ towards :: Integral a => a -> a -> [a]
 towards destination x =
   if destination == x then
     []
+  -- special case for 1-bit numbers
+  else if destination == 0 && x == 1 then
+    [0]
   else
     let
       -- Halve the operands before subtracting them so they don't overflow.