More, better encryption schemes

This release is a fairly large extension of the cryptographic systems in Infinitree. Support for multiple crypto schemes have been added, including Yubikeys, and split key encryption through libsodium. These are gated through the yubikey and cryptobox features, respectively. Check out the documentation for details.

The default encryption mode has also changed, and will transparently upgrade existing trees to the newer construct. This construct provides more flexibility and a more robust crypto implementation, that potentially re-used nonces and may have been vulnerable to partitioning oracle attacks, as noted here.

In addition, the new crate, infinitree-backends, has been added to track non-essential backends to be used by infinitree. The API should be easy to grasp even though documentation is mostly lacking.