* Rationalize input validation exceptions and assertions
... under a single ThrowHandler concept. This makes it possible to set
an alternative ThrowHandler in our C# bindings that properly translates
input validation exceptions to the correct .NET exception types.
* Rename api/input_validation.h to util/exception.h
* Reimplement util/hard_assert.h in terms of util/exception.h
* Remove stale references to DatabaseId
* Have Firestore take the DatabaseId directly
* Allow tests to default the database implicitly
* Have FSTSerializerBeta take DatabaseId by value
* Have FSTUserDataConverter take DatabaseId by value
* Make FIRFirestore.databaseID a const reference
* Make FSTFirestoreClient.databaseID a const reference
* Have FSTReferenceValue take DatabaseId by value
* Have DatabaseInfo take DatabaseId by value
* Have Serializer take DatabaseId by value
* Fix C++ ReferenceValue
* Take DatabaseId by value
* Reverse order to match Android
* Rename `reference` to `key`
* Add versioning to Functions and convert to FIRLibrary
* Convert Firestore to FIRLibrary
* Point travis to FirebaseCore pre-release for its deps
* Update user agent strings to match spec
* replace all references to `FSTDispatchQueue` and `FSTTimerID` with their C++ equivalents;
* remove `FSTDispatchQueue` class and related tests;
* in method argument names, replace `workerDispatchQueue` with just `workerQueue`, more similar to other platforms;
* move `Executor` and derived classes out of `internal` namespace.
* Add a MakeStringView benchmark
Turns out this is worse than just creating a std::string:
BM_MakeString 29 ns 29 ns 189065849
BM_MakeStringView 59 ns 59 ns 93013367
* Convert database_id to take std::string
* Convert DatabaseInfo to take std::string
* Convert User to take std::string
* Convert Token to take std::string
* Convert MakeStringView([NSString stringWithFormat:]) to StringFormat
* Move Firestore to Auth interop library.
* Fixed TODO with no name.
* Extract interop into separate class.
* Further Firestore interop changes.
* Move auth instance to constructor of CredentialsProvider.
* Resolved travis failures.
* Create separate queue for FIRApp instances, remove dead test code.
* Add missing <string> import.
Gil