[Rd] Changes to Rd handling in R-devel

[Duncan Murdoch]

I've just committed some fairly big changes to R-devel. 
  - There's a new tag \Sexpr which allows R code to be embedded within 
the Rd file, similar to Sweave,  \RdOpts
    corresponds to \SweaveOpts.
  - The parser now mainly issues warnings, rather than errors, in case 
of syntax errors.  It throws away a few tokens and tries to restart.  
This should let you see most of your errors in one pass, rather than one 
at a time.  (Don't ignore the warnings -- you've lost some of your file 
if you get them!)  (Romain:  yes, this is done using the special "error" 
token in bison.)
  - There are also a number of bug fixes to the rendering code (e.g. 
Rd2HTML).  I'm hoping to switch over soon to the new parser being used 
to produce all man pages.

I need to write up more about \Sexpr, but the basic idea is that the 
format is

\Sexpr{R code}

or

\Sexpr[options]{R code}

Currently the R code is not executed unless you do it manually, but once 
we're using this engine for man pages, you'll be able to choose when to 
execute the code:  build time, install time, or render time.

I hope these changes don't introduce any bugs in old Rd files, but if 
they do, please let me know and I'll fix them.  (Not today; I'll be 
travelling.)

Duncan Murdoch