[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/unify-types.cpp

@@ -264,7 +264,7 @@ std::ostream& TypeExpr::print(std::ostream& os, int lex_level) const {
           return os << "slice";
         case _Builder:
           return os << "builder";
-        case _Cont:
+        case _Continutaion:
           return os << "cont";
         case _Tuple:
           return os << "tuple";