* 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.
* Move ServerTimestamp into its own file
* Rewrite ServerTimestamp as a property wrapper
Add Swift4ServerTimestamp that behaves as before.
* Make ServerTimestampWrappable dictate timestamp conversion.
This makes the @ServerTimestamp property wrapper completely open to
extension by end users too.
Gil