Ask rustfmt to make our imports consistent

These are nightly-only options: so keep them in a separate file.

When it sees unstable features, stable rustfmt gives a diagnostic like:

> Warning: can't set `imports_granularity = Module`, unstable features are only available in nightly channel.
> Warning: can't set `group_imports = StdExternalCrate`, unstable features are only available in nightly channel.

But: _does_ otherwise format the files and exit non-zero.  However, this is noisy.

We arrange that `cargo +nightly fmt-unstable` also does the right thing.

.cargo/config.toml

@@ -0,0 +1,3 @@
+[alias]
+
+fmt-unstable = ["fmt", "--", "--config-path", ".rustfmt.unstable.toml"]

.rustfmt.unstable.toml

@@ -0,0 +1,6 @@
+# keep in sync with .rustfmt.toml
+chain_width = 40
+
+# format imports
+group_imports = "StdExternalCrate"
+imports_granularity = "Module"