[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/pure-functions.tolk

@@ -13,8 +13,8 @@ fun f_pure2(): int {
 fun get_contract_data(): (int, int) {
     var c: cell = getContractData();
     var cs: slice = c.beginParse();
-    cs~loadBits(32);
-    var value: int = cs~loadUint(16);
+    cs.loadBits(32);
+    var value: int = cs.loadUint(16);
     return (1, value);
 }