Merge: Calculator: add scientific features and nicer adaptations for Android and iOS

This is a long overdue update to the calculator which is the reference example for the _app.nit_ framework.

This PR adds scientific operations and constants to the calculator, directly in the logic, and only as a refinement of the UI. There is also new aesthetic adaptations for Android and iOS.

The calculator now uses Unicode operators like ÷ and ×, and internally a string instead of a `FlatBuffer` for simplicity. This change revealed errors in FFI wrappers where `to_s_with_copy` should have been used.

Pull-Request: #2066
Reviewed-by: Jean Privat <jean@pryen.org>