This PR makes Clippy deny all warnings across the workspace.
We now enumerate all of the rules that have violations and temporarily
allow them, with the goal being to drive the list down over time.
On Windows we don't yet use `--deny warnings`, as the Windows build
still has some warnings.
Release Notes:
- N/A
This PR sets up a `cargo xtask clippy` command for running `cargo
clippy` with our defined set of options.
The intent is to make this easier to manage as we start enabling more
Clippy rules.
Release Notes:
- N/A
