Add Matchable to the parents of Null in explicit nulls

[noti0na1]

Matchable should be a parent of Null in explicit nulls.

Fix the errors created by @bishabosha:

$ scala -Yexplicit-nulls
scala> def foo[T <: Matchable](t: T) = t match { case t: Null => () }
1 |def foo[T <: Matchable](t: T) = t match { case t: Null => () }
  |                                               ^
  |        this case is unreachable since type T and class Null are unrelated

scala> def foo[T <: Matchable](t: T) = t match { case null => () }                                               
1 |def foo[T <: Matchable](t: T) = t match { case null => () }
  |                                               ^^^^
  |Values of types Null and T
  |
  |where:    T is a type in method foo with bounds >: (?1 : Null) and <: Matchable
  | cannot be compared with == or !=

[sjrd]

That seems correct to me as a quick fix for now.

That said, I am still utterly convinced that we need to make Null extend AnyVal, which would also solve that problem by construction.