TO DO: serialization for upcoming R7 package

[traversc]

Two changes:

1. There will be a marhsall generic that will wrap R7 objects into plain R data. This could benefit from calling marhsall internally.

2. R7 uses S4 objects as the base for R7 that lack the S4 flag. Currently, qs assumes all S4 objects carry the S4 flag.

https://github.com/RConsortium/OOP-WG