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.