[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-mutate-15.tolk

@@ -0,0 +1,12 @@
+fun asdf(mutate cs: slice) {}
+
+fun main(cs: slice) {
+    cs.asdf();
+}
+
+/**
+@compilation_should_fail
+@stderr function `asdf` mutates parameter `cs`
+@stderr consider calling `asdf(mutate cs)`, not `cs.asdf`()
+@stderr alternatively, rename parameter to `self` to make it a method
+ */