Implement `From<Box<dyn Diagnostic>>` for `MietteDiagnostic`

[ManicMarrc]

This could be useful for creating and modifying a dynamic diagnostic using an already existing diagnostic.

[milesj]

Did you ever resolve this in anyway? I'm looking into boxing errors to reduce enum sizes, and something like this would help being built-in.

[zkat]

I'll take a PR for it if any of y'all want to toss one over :)