[Tolk] AST-based semantic analysis, get rid of Expr

This is a huge refactoring focusing on untangling compiler internals
(previously forked from FunC).
The goal is to convert AST directly to Op (a kind of IR representation),
doing all code analysis at AST level.

Noteable changes:
- AST-based semantic kernel includes: registering global symbols,
  scope handling and resolving local/global identifiers,
  lvalue/rvalue calc and check, implicit return detection,
  mutability analysis, pure/impure validity checks,
  simple constant folding
- values of `const` variables are calculated NOT based on CodeBlob,
  but via a newly-introduced AST-based constant evaluator
- AST vertices are now inherited from expression/statement/other;
  expression vertices have common properties (TypeExpr, lvalue/rvalue)
- symbol table is rewritten completely, SymDef/SymVal no longer exist,
  lexer now doesn't need to register identifiers
- AST vertices have references to symbols, filled at different
  stages of pipeline
- the remaining "FunC legacy part" is almost unchanged besides Expr
  which was fully dropped; AST is converted to Ops (IR) directly

tolk/pipe-constant-folding.cpp

@@ -0,0 +1,68 @@
+/*
+    This file is part of TON Blockchain source code.
+
+    TON Blockchain is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License
+    as published by the Free Software Foundation; either version 2
+    of the License, or (at your option) any later version.
+
+    TON Blockchain is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with TON Blockchain.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "tolk.h"
+#include "ast.h"
+#include "ast-replacer.h"
+
+/*
+ *   This pipe is supposed to do constant folding, like replacing `2 + 3` with `5`.
+ *   It happens after type inferring and validity checks, one of the last ones.
+ *
+ *   Currently, it just replaces `-1` (ast_unary_operator ast_int_const) with a number -1.
+ *   More rich constant folding should be done some day, but even without this, IR optimizations
+ * (operating low-level stack variables) pretty manage to do all related optimizations.
+ * Constant folding in the future, done at AST level, just would slightly reduce amount of work for optimizer.
+ */
+
+namespace tolk {
+
+class ConstantFoldingReplacer final : public ASTReplacerInFunctionBody {
+  static V<ast_int_const> create_int_const(SrcLocation loc, td::RefInt256&& intval) {
+    auto v_int = createV<ast_int_const>(loc, std::move(intval), {});
+    v_int->assign_inferred_type(TypeExpr::new_atomic(TypeExpr::_Int));
+    v_int->assign_rvalue_true();
+    return v_int;
+  }
+
+  AnyExprV replace(V<ast_unary_operator> v) override {
+    parent::replace(v);
+
+    TokenType t = v->tok;
+    // convert "-1" (tok_minus tok_int_const) to a const -1
+    if (t == tok_minus && v->get_rhs()->type == ast_int_const) {
+      td::RefInt256 intval = v->get_rhs()->as<ast_int_const>()->intval;
+      tolk_assert(!intval.is_null());
+      intval = -intval;
+      if (intval.is_null() || !intval->signed_fits_bits(257)) {
+        v->error("integer overflow");
+      }
+      return create_int_const(v->loc, std::move(intval));
+    }
+    // same for "+1"
+    if (t == tok_plus && v->get_rhs()->type == ast_int_const) {
+      return v->get_rhs();
+    }
+
+    return v;
+  }
+};
+
+void pipeline_constant_folding(const AllSrcFiles& all_src_files) {
+  replace_ast_of_all_functions<ConstantFoldingReplacer>(all_src_files);
+}
+
+} // namespace tolk