Add support for asymmetrical padding

[blefevre]

This adds support for asymmetrical padding allowing each of the four sides to have a different padding value if desired.

The padding function now accepts a type of Into<Padding> which is implement by u16, [u16;2] and [u16;4].
This allows paddings to specified in the following ways:

// Non breaking, keeps current behavior of 20px on all sides
Row::new().padding(20);

// 10px on the top and bottom, 20px on the left and right, 
Row::new().padding([10, 20]);

// 5px on the top, 10px on the right, 15px on the bottom, 20px on the left
Row::new().padding([5, 10, 15, 20]);

Closes: #519

[hecrj]

This is great! Thank you 🎉

I made some small changes here and there and... I believe we can finally ship this! 🚢