Widget should own its renderer

When extending a widget (e.g., to use a template with a different rendering engine), we're usually stumbling upon the fact that the renderer used to render a template is an instance variable on the Field. Consider this code example:

class ColorChooserWidget(TextInputWidget):
    template = "forms/color_chooser.jinja2"

    def serialize(self, field, cstruct, **kw):
        original_renderer = field.renderer
        field.renderer = lambda template, **kwargs: render(self.template, kwargs)
        rendered = super(ColorChooserWidget, self).serialize(field, cstruct, **kw)
        field.renderer = original_renderer
        return rendered

This works, but I do not see why the renderer would have to be set on the field. It would be much more sensible if the widgets would go through a self.render() which is easy to override.

Start by tracing how widgets serialize templates and how the field.renderer instance variable is used. Compare the current temporary renderer replacement with the proposed widget-level render hook; done means a widget can override rendering without mutating the field renderer, with behavior covered by tests.