[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

crypto/smartcont/tolk-stdlib/tvm-lowlevel.tolk

@@ -21,9 +21,5 @@ fun transformSliceToContinuation(s: slice): continuation
 
 /// Moves a variable or a value [x] to the top of the stack.
 @pure
-fun stackMoveToTop<X>(x: X): X
+fun stackMoveToTop<X>(mutate self: X): void
     asm "NOP";
-
-/// Mark a variable as used, such that the code which produced it won't be deleted even if it is not impure.
-fun stackMoveToTopImpure<X>(x: X): void         // todo needs to be deleted, check verified contracts
-    asm "DROP";