luksy: offline encryption/decryption using LUKS formats

luksy implements encryption and decryption using LUKSv1 and LUKSv2 formats. Think of it as a clunkier cousin of gzip/bzip2/xz that doesn't actually produce smaller output than input, but it encrypts, and that's nice.

• The main goal is to be able to encrypt/decrypt when we don't have access to the Linux device mapper. Duplicating functions of cryptsetup that it can perform without accessing the Linux device mapper is not a priority.
• If you can use cryptsetup instead, use cryptsetup instead.