Changed the assertions of the recursion step of decomp to a form more suitable for discharging proof obligations.
The prover could not infer the criteria on the fields of the records of ATOMS. Added them explicitly to the precondition of the put_atom-operation.
Discharged all proof obligations for Decompressor.
Discharged all but one of the current proof obligations for Encoder. Note, though, that the invariant is not complete yet.