Update use declarations formatting options.

As decided in rust-lang/compiler-team#750.

Use declarations are currently wildly inconsistent because rustfmt is
quite unopinionated about how they should be formatted. The
`rustfmt.toml` additions makes rustfmt more opinionated, which avoids
the need for any decision when adding new use declarations to a file.

This commit only updates `rustfmt.toml`. The next commit will do the
reformatting.

rustfmt.toml

@@ -2,6 +2,8 @@
 version = "Two"
 use_small_heuristics = "Max"
 merge_derives = false
+group_imports = "StdExternalCrate"
+imports_granularity = "Module"
 
 # Files to ignore. Each entry uses gitignore syntax, but `!` prefixes aren't allowed.
 ignore = [