Add a "not None" type

[KotlinIsland]

I regularly find it annoying that there is no way to type something as "anything, except None".

In Kotlin the root type Any is split from null, and the union of those is denoted Any?

In TypeScript the root type unknown includes null and undefined, but those types are distinct from the other basal types object, number etc. {} can be used to refer to any type except null(and undefined)

I think a Not type would be overkill, but a handcrafted type that represents anything except None would based and useful.

[sobolevn]

Related #614

[srittau]

#801 already suggests a general Not type. I agree that either a general Not type or a specialized NotNone type would occasionally be useful, but all discussions about new typing features should go to typing-sig.