[CoCalc] Improved support for Rust

I'm happy to announce we improved support for Rust in CoCalc, an online platform for collaborative calculation. The stable, beta and nightly channels are available out of the box: software update 2019-11-19.

~$ rustc --version
rustc 1.39.0 (4560ea788 2019-11-04)
~$ rustc +beta --version
rustc 1.40.0-beta.2 (2a0ba54fa 2019-11-16)
~$ rustc +nightly --version
rustc 1.41.0-nightly (5c5b8afd8 2019-11-16)

Editing code files in the frame editor supports code formatting via rustfmt as well.