switch and given: remove them

Superseded by match. They support the following features not directly supported by match:
- switching on a function calls that function. Use a when guard instead.
- switch supports the continue keyword for fallthrough. I've never needed it.
- switch doesn't care when no case matches. Just use case _; () for that.
- ability to specify a default case with "default". Use case _ instead.

I kept "default" as an (unused) keyword in case I want to use it for something else in the future.