Language Server: textDocument/signatureHelp

Implements a pretty barebones function signature help mechanism in
the language server.

Users can use `Analysis::resolve_callback()` to get basic information
about a call site.

Fixes #102

crates/ra_lsp_server/src/main_loop/mod.rs

@@ -255,6 +255,7 @@ fn on_request(
         .on::<req::Completion>(handlers::handle_completion)?
         .on::<req::CodeActionRequest>(handlers::handle_code_action)?
         .on::<req::FoldingRangeRequest>(handlers::handle_folding_range)?
+        .on::<req::SignatureHelpRequest>(handlers::handle_signature_help)?
         .finish();
     match req {
         Ok((id, handle)) => {