SE-0494: Add `isIdentical(to:)` Methods for Quick Comparisons to Concrete Types

[xwu (Xiaodi Wu)]

@vanvoorden

Whether we go with isCopy(of:) or hasSameRepresentation(as:) we seem to be communicating something stronger than what we really need. We do not need — or want — to require that library maintainers return true only if other is a copy of self. It's good enough to return true if other is only "identical" to self: with the library maintainer being responsible for choosing to document in what capacity an identical representation could differ from a copy.

So, something like hasRepresentationKnownIdentical(toThatOf:)?