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.

Documentation/CodingStyle.md

@@ -666,7 +666,7 @@ T* ret = reinterpret_cast<T*>(buffer);
 ```
 
 ```cpp
-// Core::Stream::SeekableStream::tell()
+// SeekableStream::tell()
 
 // Seek with 0 and SEEK_CUR does not modify anything despite the const_cast,
 // so it's safe to do this.