[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-typing-4.tolk

@@ -0,0 +1,14 @@
+fun incNotChained(mutate self: int) {
+    self = self + 1;
+}
+
+fun cantCallNotChainedMethodsInAChain(x: int) {
+    return x.incNotChained().incNotChained();
+}
+
+/**
+The error is very weird, but nevertheless, the type system prevents of doing such errors.
+
+@compilation_should_fail
+@stderr cannot apply function incNotChained : int -> (int, ()) to arguments of type (): cannot unify type () with int
+ */