[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/invalid-declaration-6.tolk

@@ -1,8 +1,8 @@
-fun main() {
-    val imm = 10;
+get seqno(self: int) {
+    return 0;
 }
 
 /**
 @compilation_should_fail
-@stderr immutable variables are not supported yet
-*/
+@stderr get methods can't have `mutate` and `self` params
+ */