…t different args

The rules for casting `*mut X<dyn A>` -> `*mut Y<dyn B>` are as follows:
- If `B` has a principal
  - `A` must have exactly the same principal (including generics)
  - Auto traits of `B` must be a subset of autotraits in `A`

Note that `X<_>` and `Y<_>` can be identity, or arbitrary structs with last field being the dyn type.
The lifetime of the trait object itself (`dyn ... + 'a`) is not checked.

This prevents a few soundness issues with `#![feature(arbitrary_self_types)]` and trait upcasting.
Namely, these checks make sure that vtable is always valid for the pointee.

I think it's fine, but let's ask T-lang separately.

…yn _

...because it's very sketchy and causes FCWs.
In this case it *is* actually sound, but still.

I should write a better fork of anymap...

…uper trait