Add std feature to Cargo manifest

[Richard-W]

This enables use of this crate in environments where std is not
available.

[cuviper]

My first reaction is that this shouldn't matter, because proc-macros always have std available regardless of actual target. But I suspect you're encountering rust-lang/cargo#2589, where the features of build-deps end up forcing the feature in target deps too.

To be honest, I don't remember why I made that a direct dependency at all. I moved it as part of the doctest commit in #11, but it seems to work fine as just a dev-dep, which should solve your issue too.

[Richard-W]

So do you suggest moving the num-traits dep in num-derive to dev-dependencies or do you suggest that my project moves num-derive?

In the latter case this pull request can be closed.

[cuviper]

I'm suggesting to change it here.

[cuviper]

I made the change in #22, including a CI test using thumb as a no-std target. This is published as 0.2.4. Please let me know if you have any more trouble with no-std!