* perf: use darwin's clonefile syscall
This saves time and disk-space when copying files around on the same
device. File clones (aka reflinks) share backing disk blocks; they
differ from hardlinks in that inodes are not shared and the contents are
copy-on-write.
The Go standard library (as of v1.22) arranges to do a similar thing for file
copies on Linux (see: https://cs.opensource.google/go/go/+/refs/tags/go1.22.6:src/os/zero_copy_linux.go;l=53).
Unfortunately, Mac OS' more limited API is less amenable to that form of
transparent wrapping.
* Assign to named return params and use naked returns
