SWIFT-390: make bson pointer access explicit (#232)

* provide typealias for BsonPointer and MutableBsonPointer

* make all access to `bson_t` and `bson_iter_t` explicit

We often passed references of these types equally to functions
that mutate the value or not, now we use the newly introduced
`withBsonIterPointer` and `withMutableBsonIterPointer`

* `Bson` -> `BSON`

* missed two cases where we were using `bson_iter_t` by reference

Author: mbroadst