[Pitch #2] Add `isIdentical` Methods for Quick Comparisons to Concrete Types

[xwu (Xiaodi Wu)]

@scanon

If I have let a = [Double.nan], then a == a is false, but a.isIdentical(to: a) is true.

Well ackshually, it's worth pointing out that Array already makes use of a shared buffer fast path and evaluates a == a as true, and changing that behavior has been deemed a won't-fix.