Add CI `no_std` optional features integration check

[pinkforest]

Tests all no_std features with cargo hack in a target that does not have std available

Some optional features may break no_std integration that may accidentally bring std

Mirrors what we are doing for dalek:

• https://github.com/dalek-cryptography/curve25519-dalek/pull/512

[newpavlov]

The blake2 job fails because it tries to test Nightly-only features on stable channel.

[pinkforest]

Yeah I have a commit ready in push but I didn't want to burn CI before I confirm couple of things re: toolchain in zulip

As the toolchain on it seems to be pinned on some specific nightly I was wondering about

[newpavlov]

Closing in favor of #485