[Amendment] SE-0143 Conditional Conformance: Add Hashable conformance to std lib types

[ben-cohen (BenC)]

After discussion amongst the core team, we are accepting an amendment to the conditional conformance proposal that adds Hashable conformance to types such as Optional and Array when their elements are hashable. This is similar to the previous amendments, which added conformance to Equatable. An implementation scheduled for Swift 4.2 can be found here.