[o] implement Document#rewrite
[o] doc.add_stream { |st| st.bt ... }
[o] st = d.addstream
    st.tf '/helv', 12
    st.bt 5, 6, "hello"
    d.write('out.pdf') # closes the stream...
[ ] fail if insert_content or insert_font on an unclosed stream obj

[o] Document.write('-') (stdout inspiration)


[ ] stop using the st-mark thing
[ ] recompress idea? uncompress with pdftk, recompress with podoff
    smaller docs anyway... /Filter /FlatDecode

decompress:
  qpdf --qdf --object-streams=disable in.pdf out.pdf
recompress:
  qpdf in.pdf out.pdf