A generic function that renders a widget in its current state. In
order to actually render the widget, call 'render-widget' instead.

'obj' - widget object to render.

One of the implementations allows “rendering” functions. When
'render-widget' is called on a function, the function is simply
called. This allows to easily add functions to composite widgets that
can do custom rendering without much boilerplate. Similarly symbols
that are fbound to functions can be treated as widgets.

Another implementation allows rendering strings.