Don't require type when using null values

[SiXoS]

It's odd that the type must be specified when using null values.

Suggestion is to add a new type and change the typing logic so that any null type can be assigned to any other type. Support for the null type should not be needed as it should bnever be used in any type declarations. It's only a typechecking type.

When assigning null to a value that doesn't already have a type, you still need to specify type.