Pairing heap

Initial implementation of a twopass pairing heap with aux list.
Research papers linked in comments.

Where search/nsearch/last aren't needed, this gives much faster first(),
delete(), and insert().  Insert is O(1), and first/delete don't have to
walk the whole tree.

Also tested rb_old with parent pointers - it was better than the current
rb.h for memory loads, but still much worse than a pairing heap.

An array-based heap would be much faster if everything fits in memory,
but on a cold cache it has many more memory loads for most operations.

Makefile.in

@@ -95,6 +95,7 @@ C_SRCS := $(srcroot)src/jemalloc.c \
 	$(srcroot)src/mutex.c \
 	$(srcroot)src/nstime.c \
 	$(srcroot)src/pages.c \
+	$(srcroot)src/ph.c \
 	$(srcroot)src/prng.c \
 	$(srcroot)src/prof.c \
 	$(srcroot)src/quarantine.c \