Allow iter_chain to be used on no_std

[OliverUv]

Hello!

This change will allow no_std users of snafu to make use of snafu's ErrorCompat::iter_chain function. Usage remains exactly the same as before.

I could not figure out an easy way to write an automatic test for this since the cargo test harness needs std (hence the test portion of these cfg attributes which already exist in the codebase #[cfg(not(any(feature = "std", test)))])

Here is the code I have used to test whether the feature works or not: https://github.com/OliverUv/snafu_api_experiment - it is a little larger than a minimal example since it also contains a small experiment to hack in dyn trait source errors. The relevant code is L114-157 of main.rs ( https://github.com/OliverUv/snafu_api_experiment/blob/7127727fb6a2efe6ecaa85c292309dfaff9e0aa9/src/main.rs#L114 )

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[shepmaster]

I've force-pushed some changes as the crate already has an alias for Error:

https://github.com/shepmaster/snafu/blob/b552522ca92031a4450c914dce114013af383b7c/src/lib.rs#L324-L332

Can you double-check that this still works for your original case?

[shepmaster]

I've rebased on main after taking care of some tangentially-related CI failures.

[shepmaster]

I pulled down your repo and it appeared to work well with the changes I made, so I'll go ahead and merge. Thanks!