add checks to the `transmute` intrinsic to only allow "safe" transmutes

examples of unsafe transmutes:
- 32-bit values that aren't valid unicode indices casted to char
- `&T` -> `&mut T`
- null value to a `NonZero` type

examples of transmutes that should be something else (warn?)
- integer -> `*const T` (cast)
- ~~`&T` -> `*const T` (cast/coercion)~~ (linted by clippy)
- ~~`*const T` -> `&T` (should be a deref)~~ (linted by clippy)
