Mace optimisation!

* Handle all chain objects as a hnext/hprev chain.
* When removing mobjs with hnext/hprev, "repair the chain" (make the h links meet).
* Fix hidden slings, which I accidentially broke when I revamped maces the first time.
* Kill MF2_MACEROTATE. Not needed for anything anymore.
* P_MaceRotate now available to Lua to make up for it.
* Related: Made modifying hnext/hprev using Lua safer, so it keeps the reference counts in play.