#26 Check for nullness

[Tarmil]

• Add a Nullness case to the Diff type.
• The differ returns None when both values are null.
• The differ returns Some(Value(x, y)) when the type being checked is a leaf type and exactly one of the values is null. (unchanged behavior)
• The differ returns Some(Nullness(x, y)) when the type being checked is not a leaf type and exactly one of the values is null.
• In the printer:
  • in the Nullness case, simply print "is null" for one value and "is not null" for the other.
  • in the leaf case, when one of the values is null, print "is null" for the null value and "= ..." for the non-null value.