LibPDF+pdf: Enough stuff to be able to read password-protected PDFs created by macOS

[nico]

In particular, implements support for AES in LibPDF.

Also fixes an issue where the linearization detection logic would crash on inputs where the first object is an encrypted compressed stream in a not-linearized file.

[nico]

@mattco98 for LibPDF.

@alimpfard for the AES client code. I found the API a bit awkward to use: From what I understand, I have to use the span that encrypt() / decrypt() can modify to access the encrypted / decrypted data, which means I can't move the ByteBuffer I have elsewhere but have to make a copy of just the bytes in the span (?)

[nico]

My tab here had a comment about giving parse_v4_crypt a return type instead of two out params. I've done that, but now that I try to reply to that comment it tells me the comment's been deleted. LMK if you'd prefer me to change it back to the out params. I like the struct return :)

[mattco98]

My tab here had a comment about giving parse_v4_crypt a return type instead of two out params. I've done that, but now that I try to reply to that comment it tells me the comment's been deleted. LMK if you'd prefer me to change it back to the out params. I like the struct return :)

Yeah idk what GitHub did there, I started the review and then thought I hit cancel, but I guess I didn't. Anyways, I deleted it because the values coming in had some default values, but it is probably better this way regardless 😄