[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/ast.cpp

@@ -79,7 +79,7 @@ int Vertex<ast_genericsT_list>::lookup_idx(std::string_view nameT) const {
 
 int Vertex<ast_parameter_list>::lookup_idx(std::string_view param_name) const {
   for (size_t idx = 0; idx < children.size(); ++idx) {
-    if (children[idx] && children[idx]->as<ast_parameter>()->get_identifier()->name == param_name) {
+    if (children[idx] && children[idx]->as<ast_parameter>()->param_name == param_name) {
       return static_cast<int>(idx);
     }
   }
@@ -96,8 +96,64 @@ int Vertex<ast_parameter_list>::get_mutate_params_count() const {
   return n;
 }
 
-void Vertex<ast_import_statement>::mutate_set_src_file(const SrcFile* file) const {
-  const_cast<Vertex*>(this)->file = file;
+// ---------------------------------------------------------
+// "assign" methods
+//
+// From the user's point of view, all AST vertices are constant, fields are public, but can't be modified.
+// The only way to modify a field is to call "mutate()" and then use these "assign_*" methods.
+// Therefore, there is a guarantee, that all AST mutations are done via these methods,
+// easily searched by usages, and there is no another way to modify any other field.
+
+void ASTNodeExpressionBase::assign_inferred_type(TypeExpr* type) {
+  this->inferred_type = type;
+}
+
+void ASTNodeExpressionBase::assign_rvalue_true() {
+  this->is_rvalue = true;
+}
+
+void ASTNodeExpressionBase::assign_lvalue_true() {
+  this->is_lvalue = true;
+}
+
+void Vertex<ast_identifier>::assign_sym(const Symbol* sym) {
+  this->sym = sym;
+}
+
+void Vertex<ast_self_keyword>::assign_param_ref(const LocalVarData* self_param) {
+  this->param_ref = self_param;
+}
+
+void Vertex<ast_function_call>::assign_fun_ref(const FunctionData* fun_ref) {
+  this->fun_maybe = fun_ref;
+}
+
+void Vertex<ast_dot_method_call>::assign_fun_ref(const FunctionData* fun_ref) {
+  this->fun_ref = fun_ref;
+}
+
+void Vertex<ast_global_var_declaration>::assign_var_ref(const GlobalVarData* var_ref) {
+  this->var_ref = var_ref;
+}
+
+void Vertex<ast_constant_declaration>::assign_const_ref(const GlobalConstData* const_ref) {
+  this->const_ref = const_ref;
+}
+
+void Vertex<ast_parameter>::assign_param_ref(const LocalVarData* param_ref) {
+  this->param_ref = param_ref;
+}
+
+void Vertex<ast_function_declaration>::assign_fun_ref(const FunctionData* fun_ref) {
+  this->fun_ref = fun_ref;
+}
+
+void Vertex<ast_local_var>::assign_var_ref(const Symbol* var_ref) {
+  this->var_maybe = var_ref;
+}
+
+void Vertex<ast_import_statement>::assign_src_file(const SrcFile* file) {
+  this->file = file;
 }
 
 } // namespace tolk