Cells should be immutable about aurae

[future-highway]

Reviewing the code as it is now, we are almost there. I think we can make this possible in 2 steps:

1. Create a 3rd variant on CellState called Freed that change the fn free to set that state instead of going back to Unallocated (this prevents Cell.allocate from doing anything in Freed also.
2. We also want to make sure that we don't accidentally make any of Cell's fields public. We can do this 2 ways:
   a. Write a comment and hope it gets paid attention too
   b. Obviously a macro that will override any attempt to set pub or pub(crate) (there may be a crate for this if we want this choice, or it shouldn't be hard to write)

[future-highway]

#180 implements step 1 and step 2a.

[dmah42]

i'm happy with this until we find out it was insufficient and the comment ended up being a sign not a cop.