Improve Padding ergonomics

We expose free functions for creating a `Padding`
and methods with the same name to modify its fields.

core/src/lib.rs

@@ -20,6 +20,7 @@ pub mod keyboard;
 pub mod layout;
 pub mod mouse;
 pub mod overlay;
+pub mod padding;
 pub mod renderer;
 pub mod svg;
 pub mod text;
@@ -35,7 +36,6 @@ mod color;
 mod content_fit;
 mod element;
 mod length;
-mod padding;
 mod pixels;
 mod point;
 mod rectangle;