implement PDF 1.5+ Object Streams

[dwarring]

Most likely in PDF::IO::Serializer.

These do result in smaller PDF files.

Potentially also of benefit to serialization, in reducing peak memory size and number of objects, but only if we can the object streams built early enough.

[dwarring]

We can't serialize to an Object-Steam until all indirect references have been resolved. One initial thought is to first include just leaf indirect objects that don't have indirect references.