[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-self-5.tolk

@@ -0,0 +1,15 @@
+fun increment(mutate self: int): self {
+    self = self + 1;
+    return self;
+}
+
+fun cantReturnAnotherSelf(mutate self: int): self {
+    self = self + 1;
+    var x = 0;
+    return x.increment();
+}
+
+/**
+@compilation_should_fail
+@stderr invalid return from `self` function
+ */