Expose crypto in the File API.

[hainesr]

See #267 for an old implementation that needs to be updated now.

[sandstrom]

@hainesr I'd close this one. Encrypted Zip files are rarely used. So I'm not convinced it's worth the added work and maintenance cost.

99% of cases when you want to protect data a zipfile with encryption, you'd just wrap the encryption around the zip.

[hainesr]

I would like to do this at some point and I've given it a suitably nebulous milestone, so I don't mind this one hanging around. 3.x isn't waiting on this, that's for sure 😅