Transferring ownership to subdirectories

[bgs99]

It would be nice to have a solution for converting a subdirectory of the tmp::directory into a new tmp::directory that is not going to be deleted when its parent is destroyed.

Currently, the options are:

1. tmp::directory::copy the subdirectory - which forces a recursive copy
2. tmp::directory dir; fs::rename(tmpdir / "subdirectory", dir); - which is somewhat verbose and additionally requires handling of the cross-filesystem moves.
3. Wrapping the subdirectory with the original tmp::directory to prevent its deletion - which has 0 overhead and does not seem to have hidden problems, but is extremely verbose.

I think the method analogous to tmp::directory::copy which moves the filesystem tree into the new temporary directory would be a good solution for this:

tmp::directory subdirectory = tmp::directory::moveFrom(tmpdir / "subdirectory");