This removes all remnants of `@` pointers from rustc. Additionally, this removes
the `GC` structure from the prelude as it seems odd exporting an experimental
type in the prelude by default.
Closes#14193
[breaking-change]
- we now distinguish bound/free parameters (see region-param
test case for why this is necessary)
- we also track bounds on region variables
- also, restructure fold_ty() to have multiple variants without
duplication instead of one overloaded folder. This also allows
for using block functions.
In the writeback phase, the typechecker now checks that it isn't
replacing a type variable T with a type that contains T. It
also does an occurs check in do_autoderef in order to avoid
getting into an infinite chain of derefs.
I'm a bit worried that there are more places where the occurs
check needs to happen where I'm not doing it now, though.
Closes#768
