Eliminate the pub field in Atom

[kmcallister]

Right now the data field is public for use by the atom!() macro. You can violate memory safety by putting garbage in this field. We should make it private somehow, or get support into rustc for unsafe fields.

[SimonSapin]

I’ve started https://internals.rust-lang.org/t/matching-on-struct-patterns-with-private-read-only-fields/3499 to discuss what can be done at the language level to help with this. The lead that sounds most likely to me is nrc’s comment:

I certainly plan to make privacy hygiene a thing for macros, not sure when that will happen though

In the meantime, #160 renames the field to unsafe_data and hides it in rustdoc.