rust: move rustfmt.toml to repo root so it can be used by `hg fix` `hg fix` runs the formatters from the repo root so it doesn't pick up the `rustfmt.toml` configs we had in each the `hg-core`, `hg-cpython`, and `rhg` packages, which resulted in warnings about `async fn` not existing in Rust 2015. This patch moves the `rustfmt.toml` file to the root so `hg fix` will use it. By putting the `rustfmt.toml` file in a higher-level directory, it also applies to the `chg` and `hgcli` packages. That makes `test-check-rust-format.t` fail, so this patch also applies the new formatting rules to those packages. Differential Revision: https://phab.mercurial-scm.org/D9142

--- a/tests/test-check-code.t	Wed Sep 30 18:07:21 2020 +0530
+++ b/tests/test-check-code.t	Thu Oct 01 09:09:35 2020 -0700
@@ -69,6 +69,7 @@
   hg
   hgeditor
   hgweb.cgi
+  rustfmt.toml
   setup.py
 
 Prevent adding modules which could be shadowed by ancient .so/.dylib.