Switch sss implementation to jwerle/shamirs-secret-sharing
dsprenkles/sss-node is designed for node-js and is not currently maintained.
This MR switches to https://github.com/jwerle/shamirs-secret-sharing for browser compatibility, and also switches from sodium-native to sodium-universal.
This does mean longer shares - for a 32 bytes secret, shares are 82 bytes rather than 33 bytes with Sprenkles' implementation.