Version 0.9.0

• Renamed PdfWriter type to Pdf(Breaking change)
• Added Chunk type for writing two things at once
• Moved top-level writers from PdfWriter to Chunk, Pdf now derefs to Chunk (Breaking change)
• Added Chunk::renumber and Chunk::renumber_into for changing indirect reference IDs after writing
• Added Pdf::set_file_id function for setting the file ID
• Added Ref::{next, bump} for simple ID allocation
• Marked Ref::{new, get} as const
• Improved writing of PDF strings
• Removed Type trait (it wasn't exported before, but used in bounds)