[Rd] tab-complete for non-syntactic names could attempt backtick-wrapping

[Ivan Krylov]

There turn out to be a few more things to fix.

One problem is easy to solve: vapply() needs a third argument
specifying the type of the return value. (Can we have unit tests for
tab completion?)

The other problem is harder: `comps` defaults to an empty string, and
you can't have a symbol consisting of an empty string, because this
value is internally reserved for missing function arguments. I think
you can return(paste0(prefix, op)) if length(comps) == 0L, but this is
still somewhat worrying. R tries to prevent empty names, so I wouldn't
expect specialOpCompletionsHelper() to return an empty string, but I
can't prove it right now.

On the other hand, x$'a string' is the same as x$`a string`. Could we
just drop as.name() and keep the return value being a string literal?
I'm not sure about this, either.

-- 
Best regards,
Ivan