[Proposal] add is identical methods to concrete types

[vanvoorden]

Many types in Foundation are “copy-on-write” data structures. These types present as value types, but can leverage a reference to some shared state to optimize for performance. When we copy this value we copy a reference to shared storage. If we perform a mutation on a copy we can preserve value semantics by copying the storage reference to a unique value before we write our mutation: we “copy” on “write”.

This means that many types in Foundation already have some private reference that can be checked in constant-time to determine if two values are identical. Because these types copy before writing, two values that are identical by their shared storage must be equal by value.