Performance suggestion: Make the mapper in oneWayMap bindings lazy or memoized

[cmeeren]

As far as I can see, oneWayMap represents an optimization over oneWay in the sense that PropertyChanged will not be triggered unless the return value of getter has changed. However, if there are multiple bindings from XAML to a single oneWayMap (e.g. one to display text and another to select a color based on the text using an IValueConverter), then the mapper will still be called several times.

It seems to me there should be a fairly simple fix wherein Elmish.WPF could use lazy to ensure that until the getter output has changed (or, if simpler, until the model is updated, though that's a lesser optimization), mapper is called at most once.

I haven't been able to figure out how, though. Any thoughts?

[cmeeren]

Closed by #48