[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/use-before-declare.tolk

@@ -1,7 +1,7 @@
 fun main(): int {
     var c: cell = my_begin_cell().storeInt(demo_10, 32).my_end_cell();
     var cs: slice = my_begin_parse(c);
-    var ten: int = cs~loadInt(32);
+    var ten: int = cs.loadInt(32);
     return 1 + demo1(ten) + demo_var;
 }