Implement crypto.encrypt_file

[hulto]

Is your feature request related to a problem? Please describe.
Eldritch doesn't have a way to encrypt a file.

Describe the solution you'd like
Create a function crypto.encrypt_file.

Should take a src file path, dst file path, and an encryption key.
Should return a status code after writing the encrypted file to disk.
Should be thoroughly tested and work on Linux, Windows, and Mac.