Various C++11 cleanups in bitworld.h; no functional change intended

One of the commits in here changes c.erase(it++) to it = c.erase(it). This probably doesn't matter much, but I consider the latter more idiomatic (see my recent blog post https://quuxplusone.github.io/blog/2020/07/08/erase-if/ ). There are some instances of c.erase(it++) in "spantree.h" as well, but I am not aware of anything that includes that file, so I'm not proposing to touch it myself.

I did fiddle around with "spantree.h" trying to make it easier to understand, but (A) I didn't get to a really "finished" state, and (B) I don't propose to merge-request this because I don't know the purpose of this code. :) You can see what I did in commit Quuxplusone/lifelib@5d265191 .