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/cmd_context/cmd_context.h

@@ -39,6 +39,7 @@ Notes:
 #include "solver/progress_callback.h"
 #include "cmd_context/pdecl.h"
 #include "cmd_context/tactic_manager.h"
+#include "cmd_context/proof_cmds.h"
 #include "params/context_params.h"
 
 
@@ -172,6 +173,7 @@ public:
     bool owns_manager() const { return m_manager != nullptr; }
 };
 
+
 class cmd_context : public progress_callback, public tactic_manager, public ast_printer_context {
 public:
     enum status {
@@ -225,6 +227,7 @@ protected:
     bool                         m_ignore_check = false;      // used by the API to disable check-sat() commands when parsing SMT 2.0 files.
     bool                         m_exit_on_error = false;
     bool                         m_allow_duplicate_declarations = false;
+    scoped_ptr<proof_cmds>       m_proof_cmds;
 
     static std::ostringstream    g_error_stream;
 
@@ -397,6 +400,8 @@ public:
     pdecl_manager & pm() const { if (!m_pmanager) const_cast<cmd_context*>(this)->init_manager(); return *m_pmanager; }
     sexpr_manager & sm() const { if (!m_sexpr_manager) const_cast<cmd_context*>(this)->m_sexpr_manager = alloc(sexpr_manager); return *m_sexpr_manager; }
 
+    proof_cmds& get_proof_cmds() { if (!m_proof_cmds) m_proof_cmds = proof_cmds::mk(m()); return *m_proof_cmds; }
+
     void set_solver_factory(solver_factory * s);
     void set_check_sat_result(check_sat_result * r) { m_check_sat_result = r; }
     check_sat_result * get_check_sat_result() const { return m_check_sat_result.get(); }