Version 0.15.0

• Added Settings struct with writing configuration
  • Added Chunk::{with_settings, with_settings_and_capacity}
  • Added Pdf::{with_settings, with_settings_and_capacity}
  • Added Content::{with_settings, with_settings_and_capacity}
• Added Content::reserve and Chunk::reserve
• Added Content::{len, as_bytes}
• Added Pdf::{finish_with_xref_stream, finish_with_xref_stream_and_filter}
• Added XRefFilter type
• The Primitive trait is now sealed (Breaking change)