Add ensure macro?

[repi]

Have you though about adding an ensure! macro as well in the same vein as the one in failure?

We use failure today and with quite a bit of ensure! which I find makes quite compact and explicit code, and has a nice naming connection to assert!.

Would you be open to adding ensure! to this crate?

[dtolnay]

Thanks @repi, good call. I would accept a PR to add an ensure macro.

[repi]

Thanks, I'll look into it.