Refactor PdfObject to Rust enum

[messense]

For example:

https://github.com/heyLu/rust-mupdf/blob/8ced6e881b34b46cc538aa3e5cf2f040ee516a3d/src/pdf.rs#L56-L139

[marioortizmanero]

This would be great. Specially if there was some way to iterate the array/dict with a more Rust-y interface, instead of with get_dict_key and similars.