Add `reorder_modules` config option.
Two things we must keep in mind when reordering modules:
1. We should not reorder modules with attributes, as doing so could
potentially break the code (e.g. `#[macro_use]`).
2. We should not reorder inline modules e.g. `mod foo { /* .. */ }`.
We should only reorder module declarations e.g. `mod foo;`.
Some open questions:
1. Should we bring modules with `pub` in front of those without `pub`
so that they stand out from others?
2. Instead of keeping modules with attributes in the same place,
can we bring them in front of others? Is this safe?
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