[Tolk] Allow cell and slice be valid identifiers

They are not keywords anymore.
> var cell = ...;
> var cell: cell = ...;
Motivation: in the future, when structures are implemented, this obviously should be valid:
> struct a { ... }
> var a = ...;
Struct fields will also be allowed to have names int/slice/cell.

tolk-tester/tests/method_id.tolk

@@ -4,6 +4,8 @@ fun foo1(): int { return 111; }
 fun foo2(): int { return 222; }
 @method_id(10)
 fun foo3(): int { return 333; }
+@method_id(11)
+fun slice(slice: slice): slice { return slice; }
 fun main(): int { return 999; }
 
 /**