The `Element::render` function now takes a constraint object. This
allows the parent element to influence how the child is rendered. This
is useful eg. to have elements take up more space than they need to fill
some space.
We also make various changes needed for pretty diffs, such as merging
styles, a new `Filled` type, and some color theming.
After cloning, we show a cool little summary with some repo stats.
We had to make some adjustments to the way visual width is calculated
due to a bug surfaced around emoji width calculation. Unfortunately
there isn't a perfect solution that works across terminals yet, so the
"variation selector 16" code-point is filtered out for now.