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/sat/sat_solver.cpp

@@ -402,6 +402,7 @@ namespace sat {
         extension::scoped_drating _sd(*m_ext.get());
         if (j.get_kind() == justification::EXT_JUSTIFICATION) 
             fill_ext_antecedents(lit, j, false);
+        TRACE("sat", tout << "drat-unit\n");
         m_drat.add(lit, m_searching);
     }
 
@@ -948,8 +949,7 @@ namespace sat {
         if (j.level() == 0) {
             if (m_config.m_drat) 
                 drat_log_unit(l, j);
-            if (!m_config.m_drup_trim)
-                j = justification(0); // erase justification for level 0
+            j = justification(0); // erase justification for level 0
         }
         else {
             VERIFY(!at_base_lvl());