Merge #901

901: Add basic support for showing fn signature when hovering r=matklad a=vipentti

This adds basic support for displaying function signature when hovering over a usage of a function. 

Additionally refactored `hover` to return `HoverResult` to ease with testing and in general to be more robust.

Co-authored-by: Ville Penttinen <villem.penttinen@gmail.com>

crates/ra_lsp_server/src/main_loop/handlers.rs

@@ -441,7 +441,7 @@ pub fn handle_hover(
     let res = Hover {
         contents: HoverContents::Markup(MarkupContent {
             kind: MarkupKind::Markdown,
-            value: info.info,
+            value: info.info.to_markup(),
         }),
         range: Some(range),
     };