Borrow checker is unsound in the presence of mutable fields

[nikomatsakis]

The current rules are not really sound. It will permit e.g. an &mut pointer to be created for a mut field found in an &T, which is aliasable. This is bad because the &mut pointer may not be the only pointer to that data.

The rewritten version I am trying to land as part of #5074 includes a fix but it's commented out because it causes compile errors in io.rs. Fixing those errors requires cleaning up object types and converting things to &mut self and I didn't want to block #5074 on those changes. (Hence this follow-on bug)

[nikomatsakis]

Since mutable fields have been removed, this is a moot point.