Change missing impl assist to use todo!() instead of unimplemented()

todo!() "Indicates unfinished code" (https://doc.rust-lang.org/std/macro.todo.html)

Rust documentation provides further clarification:

> The difference between unimplemented! and todo! is that while todo!
> conveys an intent of implementing the functionality later and the
> message is "not yet implemented", unimplemented! makes no such claims.

todo!() seems more appropriate for assists that insert missing impls.

crates/ra_syntax/src/ast/make.rs

@@ -100,6 +100,9 @@ pub fn expr_empty_block() -> ast::Expr {
 pub fn expr_unimplemented() -> ast::Expr {
     expr_from_text("unimplemented!()")
 }
+pub fn expr_todo() -> ast::Expr {
+    expr_from_text("todo!()")
+}
 pub fn expr_path(path: ast::Path) -> ast::Expr {
     expr_from_text(&path.to_string())
 }