added removeDirectory and getDirectorySize

[WebFreak001]

first I thought the file module was async using fibers etc but as it just calls phobos I made these functions just use phobos too.

[s-ludwig]

I think getDirectorySize may be a bit too high-level and too application specific for the API. rmdirRecurse on the other hand, even if on a similar abstraction level, is presumably used much more frequently, so that would still fit in. It should be named removeDirectoryRecursively or similar, though, so that nobody accidentally confuses the semantics with a plain rmdir.

For these kinds of recursive operations, which could easily block the thread for minutes, it also becomes apparent that all they really need to be moved into a thread pool at some point. Dog feeding async/Future!T would make sense, since this is currently in little use and barely tested. This doesn't need to happen as part of this PR though.