Fix builds and add CI check for no-std targets

[DrTobe]

The current master does not build for no_std targets because mbedtls-platform-supports uses once_cell::sync::OnceCell, the once_cell::sync module ist only available if the std feature is active for once_cell. Thus, the current master only builds for std targets.

This PR fixes this, but only if the tls13 feature is not active. (Edit: Not true anymore thanks to the critical-section feature in once_cell)

[Taowyoo]

The PR LGTM.
But could you provide the build command that could reproduce the error.

If possible, need to add a build check on this in CI to avoid same problem in future

[DrTobe]

If possible, need to add a build check on this in CI to avoid same problem in future

That sounds like a great idea! I will try to add a corresponding build target in the CI when I find some time to work on that.

[DrTobe]

Fixing no_std support apparently requires more than I first anticipated. The current state is not safe to merge because many types impl Sync under the false assumption that mbedtls-sys is used with the "threading" feature (thus I converted this into "Draft" state). But before I can fix this I need to understand which parts needs to be made dependent on the "threading" feature and if more changes are necessary. This requires deeper understanding of the mbedtls codebase and unraveling some of the macro magic involved. In short, this will take some more time to be finished.

[xiaoyuxlu]

@DrTobe @Taowyoo
I also have a problem with no_std build as well. #302
I suggest fixing no_std build first #303 . This will unblock the use of no_std.
The next step is to enable CI and tls13 in no_std. what do you think?

[Taowyoo]

Close this old PR because of change of default branch to main.
See more:

• [IMPORTANT] Crate status change #320

Please reopen this if have further problems or you rebased this PR onto main(this is needed on mbedtls 2.28.X)