Consider native Go implementations for KEM

[stv0g]

https://pkg.go.dev/github.com/cloudflare/circl

Kyber-512 is supported. McEliece is under development:
cloudflare/circl#378

[stv0g]

Related to rosenpass/rosenpass#91

[stv0g]

I've managed to revert the round 4 changes from cloudflare/circl#378 and pushed the round 3 version of the implementation to https://github.com/stv0g/circl/tree/classic-mceliece-round3

This unblocks me from finishing the pure-Go KEM implementation for go-rosenpass 🥳

First tests already succeeded!