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_syntax/src/ast/generated.rs

@@ -1387,7 +1387,10 @@ impl<'a> AstNode<'a> for PathExpr<'a> {
     fn syntax(self) -> SyntaxNodeRef<'a> { self.syntax }
 }
 
-impl<'a> PathExpr<'a> {}
+impl<'a> PathExpr<'a> {pub fn path(self) -> Option<Path<'a>> {
+        super::child_opt(self)
+    }
+}
 
 // PathPat
 #[derive(Debug, Clone, Copy)]