[Tolk] Get rid of ~tilda with mutate and self methods

This is a very big change.
If FunC has `.methods()` and `~methods()`, Tolk has only dot,
one and only way to call a `.method()`.
A method may mutate an object, or may not.
It's a behavioral and semantic difference from FunC.

- `cs.loadInt(32)` modifies a slice and returns an integer
- `b.storeInt(x, 32)` modifies a builder
- `b = b.storeInt()` also works, since it not only modifies, but returns
- chained methods also work, they return `self`
- everything works exactly as expected, similar to JS
- no runtime overhead, exactly same Fift instructions
- custom methods are created with ease
- tilda `~` does not exist in Tolk at all

tolk/ast.cpp

@@ -86,6 +86,16 @@ int Vertex<ast_parameter_list>::lookup_idx(std::string_view param_name) const {
   return -1;
 }
 
+int Vertex<ast_parameter_list>::get_mutate_params_count() const {
+  int n = 0;
+  for (AnyV param : children) {
+    if (param->as<ast_parameter>()->declared_as_mutate) {
+      n++;
+    }
+  }
+  return n;
+}
+
 void Vertex<ast_import_statement>::mutate_set_src_file(const SrcFile* file) const {
   const_cast<Vertex*>(this)->file = file;
 }