Begin making beatbox type-correct

This is some basic cleanup of beatbox to try and clear away the mechanical issues with its type correctness in order to expose some slightly more interesting underlying issues. Much of the remaining stuff is still noise unfortunately, mostly around "callable" not being a type in our type system and around "mixed" being a supertype but not a subtype of everything -- there's almost certainly something interesting lurking somewhere in the latter quagmire though!

I just sent you an email with more context on what I'm up to exactly -- please take a look at that, and of course feel free to grab me on IRC again if you want to chat realtime!