10809: fix: don't discard formatting of `use` lines r=Veykril a=iDawer
Use mutable syntax trees in `merge_imports`, `split_imports`. This tries to resolve#9013. But I haven't much managed to simplify code of merging.
Also resolve#9361. It reuses a use tree under the cursor so that comments+indentation are preserved. Merged trees are just appended to the end.
This touches bunch of tests. I removed the sorting of use trees as it needs a proper implementation that takes into account comments and line wrapping. I think it is rustfmt's job or at least until we get a close implementation.
Co-authored-by: iDawer <ilnur.iskhakov.oss@outlook.com>
b69eee6487