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/smt/arith_axioms.cpp

@@ -264,11 +264,12 @@ namespace arith {
         SASSERT(k1 != k2 || kind1 != kind2);
 
         auto bin_clause = [&](sat::literal l1, sat::literal l2) {
-            sat::proof_hint* bound_params = nullptr;
+            arith_proof_hint* bound_params = nullptr;
             if (ctx.use_drat()) {
-                bound_params = &m_farkas2;
-                m_farkas2.m_literals[0] = std::make_pair(rational(1), ~l1);
-                m_farkas2.m_literals[1] = std::make_pair(rational(1), ~l2);
+                m_arith_hint.set_type(ctx, hint_type::farkas_h);
+                m_arith_hint.add_lit(rational(1), ~l1);
+                m_arith_hint.add_lit(rational(1), ~l2);
+                bound_params = m_arith_hint.mk(ctx);
             }
             add_clause(l1, l2, bound_params);            
         };