`elliptic_curve::NonZeroScalar` non-zeroness guarantee

[kwantam]

Thanks a lot for the great libraries! Apologies if this has already been asked---I looked briefly and didn't see anything.

It looks like in release builds it's quite easy to construct a NonZeroScalar that equals zero (debug builds will panic).

let sk = SecretKey::new(ScalarPrimitive::ZERO);
let nzs = sk.to_nonzero_scalar();

I could easily be missing something, and of course it's easy enough to avoid with a careful check. But I think probably SecretKey::new wants to be fallible since ScalarPrimitive can be zero, while SecretKey and NonZeroScalar presumably want to be nonzero.