Add is overload for free function predicate

[filipsajdak]

The current implementation of is does not handle generic functions as predicates.

The following case works:

less_than_10_a: (i : int) -> bool = i < 10;

main: () = {
  12 is (less_than_10_a);
}

It works also when the function has a generic return type:

less_than_10_b: (i : int) -> _ = i < 10;

main: () = {
  12 is (less_than_10_b);
}

But, it does not work when the function has a generic argument. The following case does not work:

less_than_10_c: (i) -> _ = i < 10;

main: () = {
  12 is (less_than_10_c); // error
}

This change introduces additional overload for is, making the free function predicates acceptable by is (the function needs to return bool or generic type). That makes the following cases work:

less_than_10_c: (i) -> _ = i < 10;
less_than_10_d: (i) -> bool = i < 10;

main: () = {
  5 is (less_than_10_c); // works
  2 is (less_than_10_d); // works
}

Functions that return other types than bool or auto do not work.

All regression tests pass. Add a test for this case and update the regression test results.

[filipsajdak]

I am closing this one as it is covered by #701