Enable AES-256 support

[michaelrsweet]

Currently PDFio only supports RC4-40/128 and AES-128, need to also add AES-256 from PDF 2.0. Most of the supporting code is already there (including the PDFIO_ENCRYPTION_AES_256 enumeration), just need to hook the last bits up and test.

[michaelrsweet]

Sigh, since macOS still doesn't support AES-256 and PDF encryption remains a joke, deferring to "future"...