take lodepng crate from original repository

The new rustc broke lodepng, and the author had already fixed it, but my fork
was out of date, let's take the fresh version from the original repository.
disallow and remove dead code
fix and specialise pop_front_iter for libcollections reform

This also removes `pop_iter`, which was not used. The implementation is
not generic over the container any more, but specialised for `RingBuf`.
In this case I can get away with it, because it is only used for
`RingBuf`, but the generic approach was nicer in my opinion. Anyway,
there will probably be more changes in libcollections, so let’s see what
happens.
add command-line options to run in master or slave mode
replace fail! with panic!
allow specifying the port in master mode
add sender to send render results over network
implement the send task
print send task result
implement merging received images
implement pop_iter again

I just removed it from master, but it _was_ used in this branch, not
with a `RingBuf`, but with a `Vec`. So now this has a specialised
`pop_iter` for `Vec`. Again, I liked the generic one better, but this
suffices for now.