overhaul of proof format for new solver

This commit overhauls the proof format (in development) for the new core.

NOTE: this functionality is work in progress with a long way to go.
It is shielded by the sat.euf option, which is off by default and in pre-release state.
It is too early to fuzz or use it. It is pushed into master to shed light on road-map for certifying inferences of sat.euf.

It retires the ad-hoc extension of DRUP used by the SAT solver.
Instead it relies on SMT with ad-hoc extensions for proof terms.
It adds the following commands (consumed by proof_cmds.cpp):

- assume  - for input clauses
- learn   - when a clause is learned (or redundant clause is added)
- del     - when a clause is deleted.

The commands take a list of expressions of type Bool and the
last argument can optionally be of type Proof.
When the last argument is of type Proof it is provided as a hint
to justify the learned clause.

Proof hints can be checked using a self-contained proof
checker. The sat/smt/euf_proof_checker.h class provides
a plugin dispatcher for checkers.
It is instantiated with a checker for arithmetic lemmas,
so far for Farkas proofs.

Use example:
```
(set-option :sat.euf true)
(set-option :tactic.default_tactic smt)
(set-option :sat.smt.proof f.proof)
(declare-const x Int)
(declare-const y Int)
(declare-const z Int)
(declare-const u Int)
(assert (< x y))
(assert (< y z))
(assert (< z x))
(check-sat)
```

Run z3 on a file with above content.
Then run z3 on f.proof

```
(verified-smt)
(verified-smt)
(verified-smt)
(verified-farkas)
(verified-smt)
```

src/ast/ast_pp_util.cpp

@@ -175,14 +175,14 @@ std::ostream& ast_pp_util::define_expr(std::ostream& out, expr* n) {
             visit.pop_back();
             if (to_app(n)->get_num_args() > 0) {
                 out << "(define-const $" << n->get_id() << " " << mk_pp(n->get_sort(), m) << " (";            
-                out << to_app(n)->get_name(); // fixme
+                out << mk_ismt2_func(to_app(n)->get_decl(), m);
                 for (auto* e : *to_app(n)) 
                     display_expr_def(out << " ", e);
-                out << ")\n";
+                out << "))\n";
             }
             continue;
         }
-        out << "(define-const $" << n->get_id() << " " << mk_pp(n->get_sort(), m) << " " << mk_pp(n, m) << "\n";                
+        out << "(define-const $" << n->get_id() << " " << mk_pp(n->get_sort(), m) << " " << mk_pp(n, m) << ")\n";                
         m_defined.push_back(n);
         m_is_defined.mark(n, true);
         visit.pop_back();