Fix support for no_std

[kennykerr]

Rust's no_std feature doesn't actually verify that dependencies don't silently add a std dependency. It just prevents the current package from taking a direct dependency. This isn't particularly easy to test.

This PR includes a sequence of commits that will verify success/failure before completion.

Fixes: #3163
Fixes: #3164

[kennykerr]

OK, we have a green build. 2a40261

The next commit adds a duplicate panic handler. c943804

Duplicate because an indirect std dependency will bring in a panic handler and conflict.

[kennykerr]

And finally, bc4f313 avoids the indirect std dependencies and provides the actual fix to the problem. This ended up requiring a bit more work than was originally suggested by #3164.

[ChrisDenton]

Looks like a good solution. An alternative would be to fiddle with harness = false in the Cargo.toml for the test (to disable the libtest harness) but getting that wired up right might be more effort than it's worth.