updated join removal patch

Here we go again. Following Tom's advice to not insert crocks in
add_path(), I've instead introduced a noopjoin path type which ignores
its inner side. This could possibly be simplified to just a "noop"
path that doesn't even include an outer side, but the way I've done it
here doesn't really cost anything and might allow debug pprints to
print more useful information. If, in the future, we have some other
use for a noop path, we might want to reconsider, though.

Comments appreciated. This still needs some doc changes, and maybe
some regression tests, but I am not 100% sure that the basic
implementation will meet with general approval as yet, so don't want
to put the cart before the horse.

Thanks,

...Robert