Add rustfilt-like implementation

[Mark-Simulacrum]

This is essentially trivial (no argument handling etc) but it would be enough for ~all of my use cases and is ~2x faster than published rustfilt due to the demangle_stream usage, which has landed upstream but hasn't been released.

@luser, would you be up for moving ownership of rustfilt on crates.io into my / T-compiler hands? I think if so, then it would make more sense to have a clean-room implementation of it (or re-license under MIT OR Apache), and then publish under that name.

If we don't hear back my suggestion is to move forward with this -- rustfilt has a few more features, e.g., support for file paths, --hashes, and demangling arguments rather than only stdin/stdout, but in my experience the implementation here is very often sufficient. This is basically zero effort to maintain, and we can (IMO) drop it if rustfilt is more actively maintained.

[luser]

@luser, would you be up for moving ownership of rustfilt on crates.io into my / T-compiler hands? I think if so, then it would make more sense to have a clean-room implementation of it (or re-license under MIT OR Apache), and then publish under that name.

I would be happy to hand it over! It has only ever been a thin wrapper around rustc_demangle anyway, I just found it useful for my day-to-day needs.

[luser]

Please let me know what I'd need to do to transfer ownership. I'm happy to transfer the repo into the rust-lang org if that would also be helpful.

[Mark-Simulacrum]

Can you add rust-lang-owner as an owner on the crates.io? If you add me as a collaborator to the GitHub repo I can move it into rust-lang (we might archive it in favor of just moving the code into this repository).

Thanks!