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/req.rs

@@ -14,6 +14,7 @@ pub use languageserver_types::{
     CompletionParams, CompletionResponse,
     DocumentOnTypeFormattingParams,
     TextDocumentEdit,
+    SignatureHelp, Hover
 };
 
 pub enum SyntaxTree {}