Propose using optional form of as-casts only.

IMHO you should be needing `as` casts very rarely (if ever). `as?`-casts provide a safer alternative.

One notable exception might be to access bridged functionality. I.e. if you have a `myString: String` and you want to call `-length` (or whatever), then you should be doing `(myString as NSString).length()` to access the bridged functionality.

I'm all for proposing typing a few characters more if it means safer code and fewer runtime crashes.