Helper to convert SleekPadding to EdgeInsetsGeometry

[jaumard]

In some cases it can be useful to have a EdgeInsetsGeometry to provide directly under a Card::margin or Container::padding.

Something like this would be super useful:

Card(
              margin: SleekInsets.allBig.toEdgeInserts(),
              child: SleekPadding(
                padding: SleekInsets.big(),
                child: Column(
                  mainAxisSize: MainAxisSize.min,
                  children: <Widget>[_Form()],
                ),
              ),
            ),

Or having a SleekInsets extending EdgeInsetsGeometry to be able to use them directly