[Identity] Set max width to 80 characters

Summary:
The rustfmt default is 100 characters, but we use 80 in the Comm codebase

Depends on D3853

Test Plan: Ran cargo build

Reviewers: karol-bisztyga, palys-swm, jimpo, atul

Reviewed By: karol-bisztyga, atul

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3920