[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/pipe-constant-folding.cpp

@@ -88,8 +88,19 @@ class ConstantFoldingReplacer final : public ASTReplacerInFunctionBody {
     return v;
   }
 
+  AnyExprV replace(V<ast_is_null_check> v) override {
+    parent::replace(v);
+
+    // `null == null` / `null != null`
+    if (v->get_expr()->type == ast_null_keyword) {
+      return create_bool_const(v->loc, !v->is_negated);
+    }
+
+    return v;
+  }
+
 public:
-  bool should_visit_function(const FunctionData* fun_ref) override {
+  bool should_visit_function(FunctionPtr fun_ref) override {
     return fun_ref->is_code_function() && !fun_ref->is_generic_function();
   }
 };