Add PdfAttachments and PdfSignatures collections to PdfDocument.

[ajrcarey]

Provide accessor collections for document attachments and digital signatures to PdfDocument.

[ajrcarey]

FPDF_* functions relating to signatures were already available in PdfiumLibraryBindings. Added bindings for FPDF_* functions relating to embedded attachments. Added PdfAttachments, PdfAttachment, PdfSignatures, and PdfSignature objects to high level interface. Added PdfDocument::attachments(), PdfDocument::attachments_mut(), and PdfDocument::signatures() functions to PdfDocument. For the moment, all collections are read-only; Pdfium does support deleting existing attachments and creating new attachments, so PdfAttachment can gain mutation methods in a future release.

[ajrcarey]

Reinstated PdfiumError::WebSysWindowObjectNotAvailable variant removed in 0.7.16, since it is in fact needed by PdfFont when compiling to WASM. Updated documentation. Added new examples examples/attachments.rs and examples/signatures.rs. Published to crates.io as 0.7.19.