Support macOS

[ianpartridge]

It would be nice to support macOS in this library.

As macOS comes with libunwind preinstalled that could make a good backend to use.

[GeorgeLyon]

I've recently had the pleasure 😬of using libunwind for a project I've been working on, so I'd be happy to contribute this while its still paged into my brain (if this is interesting to you). From a quick glance, I don't see a canonical way to run the tests, can someone point me in the right direction?

Specifically, docker-compose seems to fail for me. Is running swift test on linux sufficient? (I'm not really sure what all the docker infrastructure is for)

failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to create LLB definition: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed

[weissi]

CC @tomerd

[GeorgeLyon]

Never mind, got this to work (I'm somewhat new to docker-compose). This is a pretty slick way of to test multiple versions of Swift/OS.