The `no_std` feature has no effect

[Liamolucko]

The no_std feature has no effect, because #![no_std] is unconditionally enabled (as it should be, since this crate has no reliance on std). It seems like this became the case in 02cc96c.

It should be removed or deprecated (probably deprecated, to avoid the breaking change), and the docs should reflect that #![no_std] is always enabled.

[Manishearth]

I think we had it in there for backcompat, but we can mark it as deprecated