Add expression form of `with`

[Technologicat]

We already have raisef and tryf, functional loops, the do[] macro to write imperative code in an expression position... the last missing major piece of the puzzle is to support invoking context managers in an expression position.

So, in a future release, let's add a withf, in the spirit of the already existing tryf.

And then update the rant in doc/design-notes.md.