Simple ADTs with a single constructor are records

[Halbaroth]

This PR fixes a bug in D_cnf that have been exposed by #1130. Before merging Enum into ADT the situation was as follows:

• simple ADT with several constructors and with at least one payload was sent to the Adt theory
• simple ADT with several constructors but no payload was sent to Enum theory
• simple ADT with a single constructor was sent to the Record theory.

The last corner case includes a corner corner case: an ADT with a single constructor without payload (so basically the unit type with an extra step...)

The D_cnf included a bug. It produced single constructors without payload of type ADT instead of type record but the check done in is_mine_symb prevented from sending it to Adt. As I removed this check, we got an assert on the following input:

(set-logic ALL)
(declare-datatype t ((Record)))
(define-fun a () t Record)
(check-sat)

After this patch, the situation is as follows:

• simple ADT with several constructors is sent to the Adt theory
• simple ADT with only one constructor is sent to the Record theory
• mutually recursive ADT whose all the types are record is sent to the Record theory