warts for isInstanceOf and asInstanceOf

Ok so here's the casting warts. I tried to catch all the synthetic usages of `isInstanceOf` and `asInstanceOf`, but the pattern matcher is a cruel magic trick so it's always possible that I missed one. I tested it on a few of my projects and it worked ok.

I added these warts to `Unsafe` but I'm thinking that may be too aggressive since it's impossible to implement `equals()` for example without casting. Maybe make a `Casting` wart that aggregates the two? Any thoughts?