firebase-ios-sdk

Rewrite SelfDocumentID as a property wrapper (#4031)

* Rewrite SelfDocumentID as a property wrapper

Make wrapped types conform to DocumentReferenceConvertible and use that
protocol to implement the conversion.

* Change DocumentReferenceConvertible to DocumentIDWrappable

* Make ServerTimestampWrappable use static functions

Use the same form as DocumentIDWrappable.

* Update extension String: ServerTimestampWrappable

* Use #if compiler, not #if swift

The FirebaseFirestoreSwift module is declared as conforming to Swift
4.0 so #if swift is still false, even when compiling with Swift 5.1.
Using #if compiler will see the actual compiler version. Property
wrappers don't depend on runtime features, so this works.

Also, allow Wrappable protocols to throw during conversions.