perf!: use Cow to avoid OsString allocations

[tilpner]

This PR eliminates many temporary allocations from unnecessary uses of OsString (they remain necessary for TreeIterator without lending iterators).

Previously, filename unescaping accounted for 43% of all total allocations, even in cases where there was nothing to unescape. This is avoided by checking for the presence of \ before, and using the original name if none was found.

comp_to_osstr was only responsible for 0.1% of allocations.

Results

The following measurements are not from a proper benchmarking setup, but merely repeated low-delta backups with heaptrack (without LTO and with raised codegen-units, on a laptop):

Total allocations reduced from 173M to 98M in my case.

These changes did not lower peak RSS significantly (4.7GiB to 4.6GiB, probably noise or slightly reduced fragmentation), which is expected for mostly temporary allocations.

Using the global allocator with glibc 2.40, there wasn't much (if any) walltime improvement (284s to 274s, measured without heaptrack), but it might be more significant on platforms with weaker allocators like musl.

[aawsome]

About the TreeIterator: I think this is one part which anyway needs some overwork: This is barely readable and too complex to understand and enhance. I wanted to implement features like allowing to use multiple different sources for a single snapshot (e.g. stdin and a local path) or to add additional sources (e.g. remote sources or using a tar as source) and almost always this complex "monster" was in the way and prevented a simple extension. (Ok, plus the parallelization used enforcing Send+Sync for many things). So IMO it's OK to keep the OsString here and tackle this when overworking the general tree handling for backups.

[aawsome]

And another thing: There is #390 which moves away from platform-dependent OsString/OsStr. But this anyway needs rebasing and some extra work; so I think we can first merge this quite small performance optimization!

[aawsome]

LGTM! Thanks a lot @tilpner