In particular, this enables HTTP/2 support in reqwest, which is a lot
more efficient.

This is a hack to fix the build on macOS, which was failing because
libnixrust.a contains compiler builtins that clash with
libclang_rt.osx.a. There's probably a better solution...

https://hydra.nixos.org/build/107473280

https://hydra.nixos.org/build/107466992

In particular, these are emitted by 'git archive' (in fetchGit).

We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.

This was the last function using a shell script, so this allows us to
get rid of tar, coreutils, bash etc.