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/IODevice.h

@@ -9,6 +9,7 @@
 
 #include <AK/EnumBits.h>
 #include <AK/Forward.h>
+#include <AK/Stream.h>
 #include <LibCore/Object.h>
 
 namespace Core {
@@ -62,12 +63,6 @@ enum class OpenMode : unsigned {
     KeepOnExec = 32,
 };
 
-enum class SeekMode {
-    SetPosition,
-    FromCurrentPosition,
-    FromEndPosition,
-};
-
 AK_ENUM_BITWISE_OPERATORS(OpenMode)
 
 class IODevice : public Object {