`Stream` will be qualified as `AK::Stream` until we remove the
`Core::Stream` namespace. `IODevice` now reuses the `SeekMode` that is
defined by `SeekableStream`, since defining its own would require us to
qualify it with `AK::SeekMode` everywhere.
Also add some tests that ensure that the input and output streams match
each other, because I can't wrap my head around what the internal
representation looks like.
