This is probably going to be controversial...
Calling end/pop in the Drop implementation causes way too many problems,
and interacts very badly with panics.
There are closure-based simpler implementations of almost everything
except parameter stacks that are still convenient to use. However, if we
end up switching to &mut Ui in most functions (very much possible in the
future!), closures will complicate matters so push/pop and begin/end
pairs are still relevant.
