AK: Move Stream and SeekableStream from LibCore

`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.

Userland/Libraries/LibCore/Forward.h

@@ -40,7 +40,6 @@ enum class TimerShouldFireWhenNotVisible;
 namespace Stream {
 class File;
 class Socket;
-class Stream;
 class BufferedSocketBase;
 }