luci-base: move luci.sys.iptparser into separate package

Since commit f6bfac21

 ("luci-mod-status: rework iptables status page"),
nothing in luci-base depends on the iptparser class anymore, so fold it
out into a separate package and let the few apps that require it depend
on the new library package.

Saves about 10K uncompressed in luci-base while the iptables status
rework enlarged the markup by roughly 5KB, saving roughly 5KB of size
overall.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>