[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-tester/tests/unbalanced_ret_nested.tolk

@@ -17,11 +17,8 @@ fun bar(x: int, y: int): (int, int) {
   }
   return (x + 1, y);
 }
-fun bar2(x: int, y: int): (int,int) {
-  return bar(x, y);
-}
 fun main(x: int, y: int): (int, int) {
-  (x, y) = bar2(x, y);
+  (x, y) = bar(x, y);
   return (x, y * 10);
 }
 /**