SWIFT-195 introduced a variety of methods for transforming documents: taking a prefix, suffix, subsequence, etc. 

Ideally, we would implement these using bson_copy_to_excluding_nonit, to avoid having to round-trip the values we want do to keep through Swift when we put them into a new document. 

However, Swift by design does not allow calling variadic C functions, as discussed in the documentation here. It does, however, support calling functions that take a va_list.

There is currently _bson_copy_to_excluding_va, but it's internal. 

Access to a variant of this method that lets us use a va_list to specify the fields to exclude would make these methods introduced in SWIFT-195 far more performant.