Configure Rustfmt

Summary:
This rustfmt.toml configuration can be added to a project root.

The main goal is to avoid to rewrite in oneline small arrays.

The normalize comments part converts /* */ blocks in // comments.

Test Plan:
Tested in D2983 between revisions 7612 and 7613.
See https://devcentral.nasqueron.org/D2983?vs=7612&id=761

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2984