Write object streams

[michaelrsweet]

PDF 1.5 and later allow value-only objects (dictionaries, arrays, etc.) to be provided in so-called object streams (type "ObjStm") which can be compressed to reduce the output file size. Cannot be used for the info, root, encryption, or length objects.

Need to abstract the value writing code to support writing to strings in addition to the PDF file.

Add objstm object number and index to objects, which is put in the cross-reference stream.