This branch has a first version of `connections' implemented
fairly closely following those in S4.

The following functions have been changed to take connections rather
than files.

cat
count.fields
dput, dump
parse
readLines
scan
sink

plus those that use them like dget, read.fwf, read.table, write, write.table.

There are new functions

open
close
seek

file
pipe  (operational only on Unix)
textConnection
isOpen
isIncomplete
pushBack
pushBackLength

print and summary methods for connections

showConnections, getAllConnections, getConnection

stdin, stdout, stderr
writeLines


Things which are under testing are output textConnections.  In
principle we could add fifos and sockets.  We also aim to add pipes
on Windows.