* 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.
* Rewrite FirestoreEncoderTests using assertThat
* Throw invalidValue for use of invalid top-level values
* Provide an message for not supported errors
* working serverTimestamp
* working FieldValue. Still need: document ref and ability to Eq of FieldValue
* DocumentReference implemented and testing works
* clean up header search path
* improving FieldValue enum comments
* run through formatting
* addressing comments
* formatted with swiftformat 0.35.7
* add missing bool->BOOL change
* undo unintended swift version change
* remove unwanted proj file change
Gil