Fix monster coalescing bug (should have known; the code looked too complicated).

More accurate sort by difficulty – now uses the average HP for that monster class. Minor code cleanups.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@3856 c06c8d41-db1a-0410-9941-cceddc491573

Created by  pauldubois  on March 24, 2008
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