This moves `Modified{Chunks,Lines}` from `src/formatting.rs` to
`src/rustfmt_diff.rs` and reexports it in `src/lib.rs`.
With this, a conversion from `Vec<Mismatch>` to `ModifiedLines` was implemented
and now this implements complementary `Display` and `FromStr`, which
simplified the previously used `output_modified` function and which allows to
parse the raw data emitted with `EmitMode::ModifiedLines`.
