Support variable-length secrets

[dsprenkels]

Note: the idea is not to change the key length, only the XSalsa20 encrypted message. We should estimate whether we want to make this the default functionality, and if so, whether it's acceptable to break backwards-compatibility with the current API.

[dsprenkels]

A draft implementation is listed in https://github.com/dsprenkels/sss/tree/rwot8-variable-length. I am personally not convinced of the security of this code.

If somebody is willing to do a proper code review, and convince me the code is safe, I may be able to merge it.

[dsprenkels]

I will mark this a wontfix after 22 Aug. I will still include a guide on how to implement the wrapper yourself (though probably as part of the sss-rs repository).

[mhamann]

I could definitely use this feature, though I probably don't have the technical C/C++ chops to review your code.

[dsprenkels]

Closing this issue as stale.