Make bytes and buffer separate packages

[jfphilbin]

1. Make Bytes a separate package independent of core
2. Test Bytes performance to ensure it is as efficient as integrated version
3. Convert all uses of Uint8List to Bytes in Core, and all other ODW packages
4. Make Buffer a separate package independent of core
5. Test Buffer performance to ensure it is as efficient as integrated version
6. Convert any uses of Uint8List as Buffer to Buffer in all ODW packages