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/euf_proof_checker.h

@@ -0,0 +1,46 @@
+/*++
+Copyright (c) 2022 Microsoft Corporation
+
+Module Name:
+
+    euf_proof_checker.h
+
+Abstract:
+
+    Plugin manager for checking EUF proofs
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2022-08-25
+
+--*/
+#pragma once
+
+#include "util/map.h"
+#include "util/scoped_ptr_vector.h"
+#include "ast/ast.h"
+
+namespace euf {
+
+    class proof_checker;
+
+    class proof_checker_plugin {
+    public:
+        virtual ~proof_checker_plugin() {}
+        virtual bool check(expr_ref_vector const& clause, app* jst) = 0;        
+        virtual void register_plugins(proof_checker& pc) = 0;
+    };
+
+    class proof_checker {
+        ast_manager& m;
+        scoped_ptr_vector<proof_checker_plugin> m_plugins;
+        map<symbol, proof_checker_plugin*, symbol_hash_proc, symbol_eq_proc> m_map;
+    public:
+        proof_checker(ast_manager& m);
+        ~proof_checker();
+        void register_plugin(symbol const& rule, proof_checker_plugin*);
+        bool check(expr_ref_vector const& clause, expr* e);
+    };
+
+}
+