v0.4.0

Improve the handling of null values.

• For leaf types such as string and System.Uri, if one of the values is null and the other isn't, then the null value is printed as XXX is null, and the non-null value is printed as XXX = <...> as before.

• For non-leaf types:

  • If both values are null, then they're considered equal and do not appear in the diff.
  • If one of the values is null, then:
    • the Diff() method returns a new union case Nullness containing both values.
    • when printed, the null value is printed as XXX is null and the non-null value is printed as XXX is not null.