explain comment more
patricia trie representation: remove unjustifiable hack
make patricia_trie's leaf type not necessarily be a pointer
fix -DPARANOID
patricia_trie: add stable refs, refactor, allow erase to delete some nodes
Put all worldblocks in the worldblock_trie_.

This makes things a little hectic (compare
  pressure_tunnel_ground -lv200
       3(global view) rrrrrr(zoom out) 9(debug view)
before and after this commit).  But it's a step
along the way to deleting those interior-only worldblocks
entirely!
patricia_trie: add erase_if_empty()
patricia_trie: simplify insert()
patricia_trie: allow inserting a blank node
test trie erasure
Change worldgen_function to a virtual class with new members.

Speed wise, it appears that I didn't make anything worse and that
spiky (the one scenario I converted to the new system) I made
moderately better even without worldblock deletion or eliding
(probably because it got to use memset in some cases).
temp hack to restore worldblock-trie behavior to prior version of sanity

(since I am still debugging trying to get the newer version working
well)