Changes to Rd handling in R-devel

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.