Add collection and document function

[michaelprichardson]

Added the collection and document function so that you can string together the collections and docs like so: firestore.collection('col1').document('doc1').collection('col2').document('doc2'). Went with document as the most of the SDKs use document instead of doc which is only used in js.

Deprecated

suspend fun <T> add(data: T, strategy: SerializationStrategy<T>, encodeDefaults: Boolean = true): DocumentReference

for

suspend fun <T> add(strategy: SerializationStrategy<T>, data: T, encodeDefaults: Boolean = true): DocumentReference

to match the parameters of the set methods