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/shell/smtlib_frontend.cpp

@@ -26,6 +26,7 @@ Revision History:
 #include "parsers/smt2/smt2parser.h"
 #include "muz/fp/dl_cmds.h"
 #include "cmd_context/extra_cmds/dbg_cmds.h"
+#include "cmd_context/proof_cmds.h"
 #include "opt/opt_cmds.h"
 #include "cmd_context/extra_cmds/polynomial_cmds.h"
 #include "cmd_context/extra_cmds/subpaving_cmds.h"
@@ -128,6 +129,7 @@ unsigned read_smtlib2_commands(char const * file_name) {
     install_subpaving_cmds(ctx);
     install_opt_cmds(ctx);
     install_smt2_extra_cmds(ctx);
+    install_proof_cmds(ctx);
 
     g_cmd_context = &ctx;
     signal(SIGINT, on_ctrl_c);