Dynamic library for native dependency

[vthemelis]

Fixes https://github.com/facebook/errpy/issues/4

I'm not sure if this is correct for a multi-target CICD though.

At the moment it just looks into the Rust build output and copies the .so or the .dylib file depending on what is present.

[facebook-github-bot]

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[vthemelis]

@jasontatton, for some more context, this was needed to be able to run pyre in ocamldebug. I added some instruction on how to do that here: https://github.com/facebook/pyre-check/issues/762

I have to say that the debugger doesn't work as well as you'd have hoped (ie it can just crash some times; probably because of the multi-processing) but I thought it was pretty valuable as it is.

[jasontatton]

Thanks for raising this. I shall make a change which will have the same effect as this PR soon (so effectively merging it). I have to do it this way as our cargo.toml files are derived automatically based on some internal config which is not surfaced within this OSS repo