Issue Description:

AnyHashable violates the transitivity requirement of Equatable:

import Foundation
let a: AnyHashable = 42 as UInt8
let b: AnyHashable = NSNumber(value: 42)
let c: AnyHashable = 42
print(a == b, b == c, a == c) // => true, true, false

Hashable requires that if two values are equal, then they must hash the same way. AnyHashable violates this, too:

let f: AnyHashable = 42.0
let g: AnyHashable = NSNumber(value: 42.0)
print(f == g, f.hashValue == g.hashValue) // => true, false

This makes AnyHashable unreliable for use in Sets or as Dictionary keys. Values inserted into a dictionary may or may not be retrievable, depending on the provenance of the lookup key. Set and Dictionary are sometimes able to detect these violations during rehashing operations; when that happens, they will (correctly!) trap with "Duplicate element found in Set"/"Duplicate key found in Dictionary".

With SE-0206 (Hashable Enhancements), we'd like to have integer types use distinct hash encodings; however, that would mean that even simple integer bridging cases would trigger the Hashable violation. (_HasCustomAnyHashableRepresentation's magical Equatable support implicitly assumes that every Swift type that bridges to the same Objective-C representation shares the same hash encoding.)