Replay FirestoreEncoder changes

[wilhuff]

Replay the changes made in the old non-conforming location and migrate
to using the FirestoreEncoder from the new location.

Changes:

f149795 (Marek Gilbert, 2 hours ago)
Migrate to encoders in third_party/FirestoreEncoder

330ef74 (wu-hui, 7 days ago)
Add @documentid equivalent to Swift Codable (#3762)

• Commit with failed optional test case.

• Integration Test and cleanup

• swiftformat

• better doc formatting

• addressing comments

• rename to SelfDocumentID

• Addressing Comments

• fix comment

e39eb40 (Hui-Wu, 3 months ago)
Allow firestore api to work with Codable objects + ExplicitNull +
Integration tests (#3261)

• Convinience api for codables

• Integration Tests

• ExplicitNull implementation

14c8d1b (Morgan Chen, 3 months ago)
Fix token fetch not invoking callback for unchanged token (#3251)

78b1828 (Hui-Wu, 3 months ago)
FieldValue and DocumentReference codable support. (#3231)

• 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

8ecec2b (Hui-Wu, 3 months ago)
Firestore swift Codable support (#3198)

FirestoreEncoder/Decoder and Timestamp/GeoPoint codable support.

30e4937 (Marek Gilbert, 2 hours ago)
Rename JSONEncoder; prep for split

8285f20 (Marek Gilbert, 7 days ago)
Pristine copy of JSONEncoder.swift

From
swiftlang/swift@3391b74
pulled with the tag swift-DEVELOPMENT-SNAPSHOT-2019-09-04-a.

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[wilhuff]

The other commits are from @wu-hui's old github handle.