Firestore customizable encoding for where clauses and update methods

[Daeda88]

Fixes #604 and trumps #605

This solution extends all update methods in Firestore to support more advanced update methods. String fields and FieldPaths can now be used interchangably and custom serializer can be passed.

documentRef.update("field" to "value, "otherField" to 1)

becomes

documentRef.update {
     // specifying build settings is optional
    buildSettings = {}

    "field" to "value"
    FieldPath("otherField") to 1
    "customField".to(CustomValueSerializer(), customValue)
}

Note that the existing vararg pairs can still be used.

Similar support has been added to where clauses as well as startAt/endAt/etc:

query.startAt {
    add(value)
    addWithStrategy(CustomValueSerializer(), customValue)
 }

There is a small regression on the existing update methods in that

public fun update(documentRef: DocumentReference, vararg fieldsAndValues: Pair<String, Any?>, buildSettings: EncodeSettings.Builder.() -> Unit)changed to public fun update(documentRef: DocumentReference, buildSettings: EncodeSettings.Builder.() -> Unit, vararg fieldsAndValues: Pair<String, Any?>) due to Kotlin not being able to figure out which update method to use.

Furthermore, I've split the Firestore tests into a few different files and extended them so they clean up all data after completion.

Lastly, I discovered bug #613 and fixed it

[Daeda88]

I still need to add Readme/some tests, but feedback would already be appreciated @nbransby as I wont be able to continue until next week

[Daeda88]

Now fully ready for review @nbransby / @Reedyuk . See description above for details of changes.