[Tolk] Nullable types T? and null safety

This commit introduces nullable types `T?` that are
distinct from non-nullable `T`.
Example: `int?` (int or null) and `int` are different now.
Previously, `null` could be assigned to any primitive type.
Now, it can be assigned only to `T?`.

A non-null assertion operator `!` was also introduced,
similar to `!` in TypeScript and `!!` in Kotlin.

If `int?` still occupies 1 stack slot, `(int,int)?` and
other nullable tensors occupy N+1 slots, the last for
"null precedence". `v == null` actually compares that slot.
Assigning `(int,int)` to `(int,int)?` implicitly creates
a null presence slot. Assigning `null` to `(int,int)?` widens
this null value to 3 slots. This is called "type transitioning".

All stdlib functions prototypes have been updated to reflect
whether they return/accept a nullable or a strict value.

This commit also contains refactoring from `const FunctionData*`
to `FunctionPtr` and similar.

tolk/ast.cpp

@@ -121,7 +121,7 @@ void Vertex<ast_reference>::assign_sym(const Symbol* sym) {
   this->sym = sym;
 }
 
-void Vertex<ast_function_call>::assign_fun_ref(const FunctionData* fun_ref) {
+void Vertex<ast_function_call>::assign_fun_ref(FunctionPtr fun_ref) {
   this->fun_maybe = fun_ref;
 }
 
@@ -129,7 +129,7 @@ void Vertex<ast_cast_as_operator>::assign_resolved_type(TypePtr cast_to_type) {
   this->cast_to_type = cast_to_type;
 }
 
-void Vertex<ast_global_var_declaration>::assign_var_ref(const GlobalVarData* var_ref) {
+void Vertex<ast_global_var_declaration>::assign_var_ref(GlobalVarPtr var_ref) {
   this->var_ref = var_ref;
 }
 
@@ -137,7 +137,7 @@ void Vertex<ast_global_var_declaration>::assign_resolved_type(TypePtr declared_t
   this->declared_type = declared_type;
 }
 
-void Vertex<ast_constant_declaration>::assign_const_ref(const GlobalConstData* const_ref) {
+void Vertex<ast_constant_declaration>::assign_const_ref(GlobalConstPtr const_ref) {
   this->const_ref = const_ref;
 }
 
@@ -149,7 +149,7 @@ void Vertex<ast_instantiationT_item>::assign_resolved_type(TypePtr substituted_t
   this->substituted_type = substituted_type;
 }
 
-void Vertex<ast_parameter>::assign_param_ref(const LocalVarData* param_ref) {
+void Vertex<ast_parameter>::assign_param_ref(LocalVarPtr param_ref) {
   this->param_ref = param_ref;
 }
 
@@ -157,23 +157,27 @@ void Vertex<ast_parameter>::assign_resolved_type(TypePtr declared_type) {
   this->declared_type = declared_type;
 }
 
-void Vertex<ast_set_assign>::assign_fun_ref(const FunctionData* fun_ref) {
+void Vertex<ast_set_assign>::assign_fun_ref(FunctionPtr fun_ref) {
   this->fun_ref = fun_ref;
 }
 
-void Vertex<ast_unary_operator>::assign_fun_ref(const FunctionData* fun_ref) {
+void Vertex<ast_unary_operator>::assign_fun_ref(FunctionPtr fun_ref) {
   this->fun_ref = fun_ref;
 }
 
-void Vertex<ast_binary_operator>::assign_fun_ref(const FunctionData* fun_ref) {
+void Vertex<ast_binary_operator>::assign_fun_ref(FunctionPtr fun_ref) {
   this->fun_ref = fun_ref;
 }
 
+void Vertex<ast_is_null_check>::assign_is_negated(bool is_negated) {
+  this->is_negated = is_negated;
+}
+
 void Vertex<ast_dot_access>::assign_target(const DotTarget& target) {
   this->target = target;
 }
 
-void Vertex<ast_function_declaration>::assign_fun_ref(const FunctionData* fun_ref) {
+void Vertex<ast_function_declaration>::assign_fun_ref(FunctionPtr fun_ref) {
   this->fun_ref = fun_ref;
 }
 
@@ -181,7 +185,7 @@ void Vertex<ast_function_declaration>::assign_resolved_type(TypePtr declared_ret
   this->declared_return_type = declared_return_type;
 }
 
-void Vertex<ast_local_var_lhs>::assign_var_ref(const LocalVarData* var_ref) {
+void Vertex<ast_local_var_lhs>::assign_var_ref(LocalVarPtr var_ref) {
   this->var_ref = var_ref;
 }