Dedup diagnostics

[jneem]

Unfortunately the lsp_types crate doesn't provide a lot of Ord impls, so I had to write some wrappers. Is there a better way? (Of course we could use hashmaps to dedup, but lsp_types also doesn't provide Hash impls)

Fixes #1882

[yannham]

I don't know of an obvious better way to do that on a foreign types (which means it's not possible to derive anything automatically either). Although I wonder if we really need the newtype wrappers boilerplate, and if we couldn't just move cmp to free-standing comparison functions instead which we then pass to e.g. sort_by. That might save a few map and unwrapping?

[jneem]

I had a try with this but it had even a little more boilerplate. It would be fine if the lsp_types types were at the "top level", but the Option<Vec<lsp_types::DiagnosticRelatedInformation>> is pretty annoying...