RFC: arbitrary transformation for renderables

[Роман Донченко]

Pierre Tardy <tardyp at gmail.com> писал в своём письме Fri, 02 Oct 2015  
00:25:36 +0300:

> Hi Roman,
>
> There is already a renderer function which is similar simplification of  
> the
> renderable pattern.
> You can do
>
> renderer(lambda _: datetime.date.today().isoformat()
> renderer(lambda b: os.path.relpath(p.getProperty('buildir'))
>
> The latter is a little bit more chars than your solution, but probably  
> more
> easy to follow.

I know this, but it doesn't work when the arguments are arbitrary  
renderables (since you have to account for the possibility that they  
return a Deferred). The brevity is nice to have, too.