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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state