Introduce case statements independently of type annotations

[masak]

It strikes me that we could perhaps eliminate NoneType and have Maybe<T> even without all of #33. (I was inspired by @raiph in thinking in this direction.)

The premise is this:

• None and NoneType are gone.
• Whenever you want to allow for a None-like value, you give something the type Maybe<Expr> or similar. On the Perl 6 level.
• We implement case statements à la Implement ADTs, and pattern matching #34.
• But without any type annotations and type inference switched on, these switch statements are mostly just syntax; they're a way to dig into a Maybe, but they don't come with any strictures. (No totality checking; no checking the cases against the surrounding type.)

I should add that this would be an experiment, to see if we like it better than the current None status quo. There is promise in the idea; it would for example address the concerns raised in #148, too.

[masak]

I liked pampy, which provides a pretty neat library-only way to pattern match. We could try porting it to 007 and see how it looks; it might be a sensible step on the way to doing pattern matching with syntax.

[masak]

I'm going to close this one; I find I don't believe strongly in getting rid of none these days. (Also, TypeScript sort of shows that it's not super-necessary either.)