Source references from the parser

Duncan Murdoch <murdoch at stats.uwo.ca> writes:

There have been a couple of requests to set the keep.source default to
FALSE (i.e. not to enable the new behaviour), so I've done that.

Thank you.  That is much appreciated.

Hopefully the code will still be exercised enough that we can have
confidence in it by April.

FWIW, I intend to set keep.source=TRUE for the packages that I work on
directly and will speak up if I encounter problems :-)