[Tolk] bool type (-1/0 int under the hood)

Comparison operators `== / >= /...` return `bool`.
Logical operators `&& ||` return bool.
Constants `true` and `false` have the `bool` type.
Lots of stdlib functions return `bool`, not `int`.

Operator `!x` supports both `int` and `bool`.
Condition of `if` accepts both `int` and `bool`.
Arithmetic operators are restricted to integers.
Logical `&&` and `||` accept both `bool` and `int`.

No arithmetic operations with bools allowed (only bitwise and logical).

tolk-tester/tests/c2_1.tolk

@@ -2,7 +2,7 @@ fun check_assoc(op: (int, int) -> int, a: int, b: int, c: int) {
   return op(op(a, b), c) == op(a, op(b, c));
 }
 
-fun main(x: int, y: int, z: int): int {
+fun main(x: int, y: int, z: int): bool {
   return check_assoc(`_+_`, x, y, z);
 }