improve a package-dependency error report from `raco setup`
raco pkg update --clone: offer advice on converting directory links
raco pkg {install,update,remove}: add --batch

Adjust defaults for `--deps` and `--multi-clone` to depend on
interactive vs. batch mode.
raco pkg show: add `-l'/`--long', and adjust the default mode

Make `raco pkg show` more readable by default by constraining the
output to 80 characters and adjusting the way some columns print.
optimizer: fix do_make_discarding_sequence

The optimizer converts (car (cons X Y)) to (begin0 X Y) and then reduces
it to (begin Y X) if X is movable.
Check that the movement is safe for space and for continuation captures.