Here’s another version of the previous patch which fixes a couple bugs
related to
1) changing the window configuration unwarrantedly
2) syntactically invalid R code (mismatched parens or quotes) could hang
   emacs

It’s easy to get R to throw an error in the mismatched-parens case, but
more difficult to recover from the mismatched-quotes one.  Thus, when
babel detects such a case it bails and signals a user-error in emacs.

(Under the present code in master, both the mismatched-parens and
mismatched-quotes cases hang emacs pending a manual C-g.)

The patch is now firmly into the territory of gross hacks, so I’d much
prefer the evaluate-based approach if it can be made palatable.