support FakeFileSystem

[benkuly]

One of the few things that is currently preventing me from switching to kotlinx-io is the lack of FakeFileSystem.

[lppedd]

We could also implement custom file systems, if FileSystem wasn't sealed. I don't recall why it was declared sealed.

[fzhinkin]

FileSystem is expected to be unstable until FS API finalization (which hasn't happened yet), by keeping it sealed we can ensure a smoother and simpler evolution cycle for it.