Support the LSP hover capability showing type info and doc comments

[JoshuaBatty]

Hovering over a token should look up the definition and show type information, and it's visibility i.e pub or not.

For functions then the function signature should be also shown.

Once doc comments have made it into the AST Nodes then we can also display the comments as formatted markdown in the hover pop-up box.

Add the following line when constructing a ServerCapabilities in server.rs hover_provider: Some(HoverProviderCapability::Simple(true))