Add profiling around add_impl_members

This intention is pretty slow for `impl Interator`, because it has a
ton of default methods which need to be substituted.

The proper fix here is to not compute the actual edit until the user
triggers the action, but that's awkward to do in the LSP right now, so
let's just put a profiling code for now.

Cargo.lock

@@ -963,6 +963,7 @@ dependencies = [
  "ra_db",
  "ra_fmt",
  "ra_hir",
+ "ra_prof",
  "ra_syntax",
  "ra_text_edit",
  "rustc-hash",

crates/ra_assists/Cargo.toml

@@ -16,6 +16,7 @@ either = "1.5"
 ra_syntax = { path = "../ra_syntax" }
 ra_text_edit = { path = "../ra_text_edit" }
 ra_fmt = { path = "../ra_fmt" }
+ra_prof = { path = "../ra_prof" }
 ra_db = { path = "../ra_db" }
 hir = { path = "../ra_hir", package = "ra_hir" }
 test_utils = { path = "../test_utils" }

crates/ra_assists/src/assists/add_missing_impl_members.rs

@@ -99,6 +99,7 @@ fn add_missing_impl_members_inner(
     assist_id: &'static str,
     label: &'static str,
 ) -> Option<Assist> {
+    let _p = ra_prof::profile("add_missing_impl_members_inner");
     let impl_node = ctx.find_node_at_offset::<ast::ImplBlock>()?;
     let impl_item_list = impl_node.item_list()?;