chore: replace `serum` -> `openbook` in code

[metaproph3t]

• replace references to serum with references to openbook

[mschneider]

Hi thank you for helping with the rename, unfortunately this change would cause a lot of unnecessary breaking changes across the ecosystem. Openbook v1 was developed under a 99% backwards compatibility mandate. Openbook v2 development (which has already started and should be open sourced today or early next week) will allow for this kind of change.

[metaproph3t]

I see, sir