Added a debounce helper

[TimLariviere]

Closes #161

Added an helper, along fix and fixf, that throttle calls to a function.
Really quick updates (like TextChanged) can make Fabulous go out of sync on Android between what it thinks the state is and what it really is, resulting in an infinite loop where Fabulous both tries to force down its values and tries to catch up with Android.
The debounce function prevents Fabulous to react too often to those updates which helps it follow the correct state, e.g.

View.Entry(
     text=model.MyEntryText,
     textChanged=debounce 250 (fun args -> dispatch (MyEntryTextChanged args.NewTextValue))
)

In this example, the message will be dispatched only if TextChanged is not triggered for 250ms since its last call.
The Text property will only be updated then.