[Pitch] Cell, ConstCell, and Volatile

Hi Evolution, as a revision to @Douglas_Gregor's Aliased*Span and Aliased*Ref proposal, we've come back with a set of different types that should compose well with the existing Span and Ref types. These types are also incredibly useful when building things like concurrency primitives like Atomic and Mutex as well as concurrent data structures. Please read the proposal and let me know what you think. Cell, ConstCell, and Volatile Proposal: SE-NNNN Authors: Alejandro Alonso, Doug Gregor Review M...