Disable implicit conversions between from `Null` and `Nothing`.

This is necessary to reject code like

     val x: Boolean = null

Without the restriction, this code would typecheck and expand to

     val x: Boolean = Predef.Boolean2boolean(null)

since `null` counts as a value of type `java.kang.Boolean`.
Merge pull request #283 from dotty-staging/fix/#291-null-primitive-conforms

Disable implicit conversions between from `Null` and `Nothing`.