Add is overload for free function predicate

Current implementation of `is` does not handle generic functions as predicates.

The following case works:
```cpp
less_than_10_a: (i : int) -> bool = i < 10;

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

It work also when function has generic return type
```cpp
less_than_10_b: (i : int) -> _ =  i < 10;

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

It does not work when function has generic argument.
The following case does not work:
```cpp
less_than_10_c: (i) -> _ = i < 10;

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

This change introduce additional overload for `is` making free function
predicates acceptable by `is` (the function needs to return
`bool` or generic type). That makes the following cases to work:
```cpp
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 type then `bool` or `auto` do not work.

Author: filipsajdak

include/cpp2util.h

@@ -894,6 +894,12 @@ inline constexpr auto is( auto const& x, auto const& value ) -> bool
     return false;
 }
 
+// Free function predicate case
+template< typename X >
+constexpr auto is( X const& x, bool (&value)(X const&) ) -> bool
+{
+    return value(x);
+}
 
 //-------------------------------------------------------------------------------------------------------------
 //  Built-in as