Make `dyn*` casts into a coercion, allow `dyn*` upcasting
I know that `dyn*` is likely not going to be a feature exposed to surface Rust, but this makes it slightly more ergonomic to write tests for these types anyways. ... and this was just fun to implement anyways.
1. Make `dyn*` into a coercion instead of a cast
2. Enable `dyn*` upcasting since we basically get it for free
3. Simplify some of the cast checking code since we're using the coercion path now
r? `@eholk` but feel free to reassign
cc `@nikomatsakis` and `@tmandry` who might care about making `dyn*` casts into a coercion