No std

[lengyijun]

can pass

cargo b
cargo b --features=no_std
cargo test

can not pass

cargo test  --features no_std

I'm not quite sure my modification is right yet. If there is some problems, I'd like to modify it more

[lengyijun]

I still feel no_std is necessary

[ogoffart]

I would also like to use this crate in a no_std context. Is this crate still maintained?

Regarding this PR, i don't think this is the way to do it. Rust feature are meant to be additive, so we cannot have a no_xxx feature, instead, we need to have positive features, like std, which is proposed in this PR, which i believe is a better approach: #47