implement BridgeStan download and module compilation on Rust

[randommm]

Implement BridgeStan download and module compilation on Rust.

Fixes issue #100

[WardBrian]

Thank you for taking this on! A few thoughts on what is here so far

[randommm]

Thank you for taking this on! A few thoughts on what is here so far

okay, all done

[WardBrian]

You may have to skip some of the compile tests on Windows for Rust, since we never unload libraries (#111) and Windows locks the file of loaded libraries

[randommm]

i see, what about Mac? it seems to give some weird error that is hard to understand, seems to be C++ toolchain related.

[WardBrian]

I’ll try to take a look this week - my guess is it has to do with the specific CI environment installing a non-native LLVM

[randommm]

there also seems to be some toolchain problem on Windows, lack of mingw, so it's unable to compile the example.

[WardBrian]

Thanks again. A few more comments below.

I'm also going to ask @aseyboldt if he would be willing to take a look at this

[randommm]

finally managed to fix the build on macos now!

[randommm]

@WardBrian @aseyboldt made the API a little more clear now regarding that bridgestan is being download, and also easier to point to a custom bridgestan dir. I think that the checksum and the lock dir would be better fit as others issues/PRs.

[WardBrian]

The following are hopefully my final comments, focusing on some edge cases in the example and tests.

I'd still like @aseyboldt do give another look over for any Rust nits I'm missing. But I think this is very good!

[WardBrian]

This looks good to me. I left two tiny questions about the tests, and I would still appreciate if @aseyboldt could have a look before merging.

Thanks again!