make dependencies optional

[cztomsik]

I'm not sure what's the idea behind feature flags (!alloc && !std) or if it's actually supposed to work (cargo build --features=alloc fails) but none of the dependencies is actually required for default (std) build so I think it would be good to mark them optional.

I did that in my fork and I can do PR (or you can just copy paste it) but I guess I've missed something because I'm not used to this project. I need to compile this on raspi so that's why I'm worried and did the fork.