Stability tracking - Rustfmt

[nrc]

Add to Rust product:

• Add to Rust repo
• agree on stability criteria
• tests running and passing on CI
• overwrite mode by default
• implement stable/unstable command line arguments (unstable options are now supported, but we need to decide which options should be stable)
• resolve 'warnings for skipped sections'

1.0 - see rust-lang/rustfmt#1860