Test `no_std` builds on a target lacking `std`

[pinkforest]

This has bitten me more times than I care to admit :)

So here is a build-test for no_std that ensures build on a target that lacks std - default target would have std available.

It also tests all the features properly when / if introduced automatically unless excluded as std e.g. getrandom / std

We use the same in curve25519-dalek

[burdges]

LGTM but can we trigger this manually?

[pinkforest]

Just needs workflow_dispatch: at top under on: