zero-copy the headers

[lidatong]

zero-copy the SlabHeader and EventQueueHeader. this is actually quite gnarly and requires:

• [ ] changing the buffer to hold only the non-event queue header
• [ ] refactoring away the Rc<RefMut> stuff (not sure if it's possible to pointer cast otherwise)
• [ ] zero-copy deserialization of the above
  • probably requires a manual combo of array_refs! and raw pointer casts (because bytemuck doesn't support DST)
  • serum-dex has a good example of how to do this for their own Queue
• [ ] updating the manually-defined schema in eventQueue.ts (for our Anchor JS client / integration tests)