Implement Ord for Severity

[bjchambers]

This would allow determining the max severity as diagnostics.iter().map(|d| d.severity).max(). As is, there is no way (except manually matching) to determine whether there were diagnostics over a certain severity (eg., to fail if an error is reported but proceed if there are only warnings).

[Johann150]

I'm not sure this is even necessary, there is already a doc-test for this that passes:

codespan/codespan-reporting/src/diagnostic.rs

Lines 12 to 19 in 8d1762e

	/// ```rust
	/// use codespan_reporting::diagnostic::Severity;
	///
	/// assert!(Severity::Bug > Severity::Error);
	/// assert!(Severity::Error > Severity::Warning);
	/// assert!(Severity::Warning > Severity::Note);
	/// assert!(Severity::Note > Severity::Help);
	/// ```

https://github.com/brendanzab/codespan/runs/3300809770#step:4:247

But sure, if that doesnt work, it can be added, since its already intended anyway.

[bjchambers]

I believe that Severity::Bug > Severity::Error works via the PartialOrd, where a return of None would correspond to false. But max requires Ord to indicate that all values are comparable.

[Johann150]

resolved with 5909730