Update the rust toolchain version to 1.85.
Note that this is not the latest, however, it is the latest available in nixpgs,
so this version is good enough for now.
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.