Add no-std and baremetal features

[geonnave]

This is an ongoing attempt at addressing #131. Heavily based on this fork. cc @chrysn @malishav

[chrysn]

Adding a baremetal and a no_std feature is contrary to the addititvity of Cargo features. If there is an actual need for std (for #131 I don't think we need an extra feature, it can be no_std and no_alloc all the time), and it's on by default now, it should become a default feature "std", so that all dependencies that are themselves no_std would use this with no-default-features, but as soon as any dependent crate needs std, std is in.

[geonnave]

Thanks for the feedback @chrysn.

Before I spend more time on this, I would like to hear what the maintainers think of addressing this issue.

[ionut-arm]

Hi!

Thanks for the patch, it looks good to me. In my opinion, making it no_std is a good step forward, but happy to hear other views too.

I wanted to ask whether it would be possible/make sense to enable it for psa-crypto too.

Also, the one thing I'd like added is some mention in the README about the new features. :)

[geonnave]

Thank you @ionut-arm.

Regarding psa-crypto, it seems to already be no-std except when using the std feature (which I think is more like what @chrysn was proposing).

In addition, I realized I need some extra build flags to be passed to mbedtls when building this in no-std mode for an embedded target (nRF52840). Unfortunately, it requires patching mbedtls. I added a patch and the required logic in build.rs, but I am still not sure how general this solution is, i.e. will others wanting to build in baremetal configuration need this same patch? (cc @malishav)