Huon Wilson worked on an implementation a year ago:
https://github.com/huonw/float
Related- there was a discussion a few years back in Reddit about future-proofing math/numbers in Rust:
https://www.reddit.com/r/rust/comments/1uy7rt/an_appeal_for_...
However, when it comes to speed, working with primitive types has gotta be faster if supported natively, so anything else anytime soon will play second fiddle.