Previously, we supported the `AtLeast` bound, which was used to describe a subset relationship in which `I: AtLeast<J>` implied that `I` as at least as restrictive as `J`. However, as described in #1866, this incorrectly models invariants as monotonic. In reality, invariants both provide guarantees but also *require* guarantees. This commit takes a step in the direction of resolving #1866 by removing `AtLeast`. Uses of `AtLeast<Shared>` are replaced by a new `Reference` trait, which is implemented for `Shared` and `Exclusive`. This serves two purposes: First, it makes it explicit what this bound means. Previously, `AtLeast<Shared>` had an ambiguous meaning, while `Reference` means precisely that an invariant is either `Shared` or `Exclusive` and nothing else. Second, it paves the way for #1183, in which we may add new aliasing invariants which convey ownership. In that case, it will be important for existing methods to add `Reference` bounds when those methods would not be sound in the face of ownership semantics. We also inline the items in the `invariant` module, which were previously generated by macro. The addition of the `Reference` trait did not play nicely with that macro, and we will likely need to go further from the macro in order to fix #1839 – this fix will likely require making aliasing invariants meaningfully different than other invariants, for example by adding an associated type. Makes progress on #1866

Audit variance of Ptr #1839

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Audit variance of `Ptr` #1839