Add support for Miri backtraces

[Aaron1011]

Companion to rust-lang/miri#1559

[alexcrichton]

Looks reasonable to me, thanks for this! I figure it's probably best to wait for the companion PR to merge first before this?

[Aaron1011]

I think the PRs should be reviewed together, but this PR should be merged first. That will allow us to add Miri tests on the backtrace-rs side once a nightly with the new Miri is available.

[alexcrichton]

Ok seems reasonable to merge then, but looks like rustfmt is needed?

[Aaron1011]

Oops, I didn't realize which repository I was commenting on. I meant to say that the Miri PR should be merged first.

[alexcrichton]

Ah makes sense! If you can ping me when the upstream PR lands I'd be happy to merge this one!

[Aaron1011]

Unfortunately, the test suite does not currently pass under Miri, so the actual content of this PR isn't being tested by CI.

However, this PR can be tested locally by running a simple program (e.g. println!("{:?}", backtrace::Backtrace::new())) under Miri, and observing the output.

I plan to do some follow-up work to get the test suite passing under Miri.

[Aaron1011]

@alexcrichton The Miri PR had landed, so this PR is ready to be merged.

[alexcrichton]

Ok, thanks!