enhancement request: Implement McCallum-Relyea exchange

[udf2457]

It would be great if you could implement high-level functions to support McCallum-Relyea exchange (as used in Clevis/Tang).

I started trying to mock-up something myself using Go's stdlib, but unfortunately it looks like the go maintainers are deprecating stdlib ability to e.g. perform elliptic curve group addition which is one of the fundamental steps, so there's scope for a third-party lib such as circl to fill the gap.

(Lots of internet descriptions of McCallum-Relyea, here's one of many).

[armfazh]

At first sight, it seems that the protocol can be implemented using CIRCL's group interface, which provides most of the operations needed to work with elliptic curves.