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` and
`compiler/rustc_codegen_cranelift/rustfmt.toml`. The next commit will do
the reformatting.

Author: nnethercote

rustfmt.toml

@@ -6,3 +6,5 @@ ignore = [
 version = "Two"
 use_small_heuristics = "Max"
 merge_derives = false
+group_imports = "StdExternalCrate"
+imports_granularity = "Module"