Dmitry Gutov wrote:
> ...but indeed, this approach could be the simpler one.

I hacked at this a bit, and found that the simpler approach also was better at 
not curving quotes that shouldn't be curved.  For example, describe-variable 
should curve the quotes in the doc string, but not in the contents of the 
variable.  I'm sure this could all be done with font locking but it's simpler to 
just edit the characters.  And I found a reasonably simple way to put it in, 
namely, to modify substitute-command-keys so that it substitutes quotes as well, 
with a simple rule that can be done in one pass with only auxiliary boolean flag.

Revised patches are attached.  The first one changes substitute-command-keys. 
The second one changes the rest of the infrastructure to match; it contains the 
bulk of the previously-proposed changes.  And the third one updates a few doc 
strings, mostly so that ` isn't turned into ‘ when it's really intended to be a 
grave accent.