Official support for no_std

Hi! I found this library after looking for a stable alternative to the unstable contracts API in core::contracts (https://doc.rust-lang.org/core/contracts/index.html)

I really like this crate and I have already used it quite a bit in std contexts. Now I'm working on a no_std project and I'm considering using contracts there as well.

As far as I can tell from the generated code, the regular contract modes seem like they should already be compatible with no_std consumers: the crate itself is a procedural macro, and the generated checks appear to rely on assert!, debug_assert!, concat!, and stringify!, which are available from core.

Would it make sense to officially support no_std by adding a CI check for it?

I think no_std is an important use case here, since contracts are closer to a language feature than to a typical library feature. In other words, they can be useful even in environments without an OS or the standard library.

Start by reviewing the generated contract code and the repository's CI configuration. Check whether the regular contract modes compile for a no_std consumer, then add a CI check if needed; done means the no_std check passes reliably.