[feature request] Release tarball and/or means to select a system-provided libcst prebuilt binary

Hi,

It seems post version 0.4, this project requires rust and cargo to build, which will connect to the internet to fetch a few extra dependencies. This doesn't mesh well with the way things are built for distributions such as GNU Guix or Nix, where the build happens in a containerized (offline) environment: the source is expected to be self-contained.

Would it be possible to produce some old-fashioned tarball archive that would contain all the vendored crates used? Or alternatively, and even preferably, being able to use a pre-built libcst packaged separately via the libcst crate could be nice (is the output of building the libcst crate a `libcst` binary?).

Thank you for your consideration!