Issues with `UnsafeCell`s

[joshlf]

See also: #251

TODO: Discuss the following:

• The fact that there's no way to "disable" interior mutability under stacked borrows, but there is under tree borrows
• Why NoCell for MaybeUninit<T> requires T: NoCell (it's the only way to ban cells today)
  • Under certain memory models (ie, not stacked borrows), MaybeUninit<T> wouldn't "expose" any of its contained Cells, and so it could be NoCell with no bounds on T
• Why our current NoCell implementation (#656) bans UnsafeCell<()> and even [UnsafeCell<u8>; 0]; what would need to change in order for these to be permitted.