Been playing with an HPKE implementation first for age and now for the stdlib.

Been playing with an HPKE implementation first for age and now for the stdlib. Ended up yak shaving generic ECDH and KEM interfaces.

If you have opinions about any of it, https://github.com/golang/go/issues/75300 is the stdlib proposal, while https://pkg.go.dev/filippo.io/hpke@v0.2.0 is the HPKE implementation, including post-quantum hybrids.

Feedback welcome!