Refactor/fix ph.

Refactor ph to support configurable comparison functions.  Use a cpp
macro code generation form equivalent to the rb macros so that pairing
heaps can be used for both run heaps and chunk heaps.

Remove per node parent pointers, and instead use leftmost siblings' prev
pointers to track parents.

Fix multi-pass sibling merging to iterate over intermediate results
using a FIFO, rather than a LIFO.  Use this fixed sibling merging
implementation for both merge phases of the auxiliary twopass algorithm
(first merging the aux list, then replacing the root with its merged
children).  This fixes both degenerate merge behavior and the potential
for deep recursion.

This regression was introduced by
6bafa6678f (Pairing heap).

This resolves #371.

include/jemalloc/internal/private_symbols.txt

@@ -82,7 +82,6 @@ arena_nthreads_dec
 arena_nthreads_get
 arena_nthreads_inc
 arena_palloc
-arena_ph_to_miscelm
 arena_postfork_child
 arena_postfork_parent
 arena_prefork
@@ -101,6 +100,12 @@ arena_ralloc_junk_large
 arena_ralloc_no_move
 arena_rd_to_miscelm
 arena_redzone_corruption
+arena_run_heap_empty
+arena_run_heap_first
+arena_run_heap_insert
+arena_run_heap_new
+arena_run_heap_remove_first
+arena_run_heap_remove
 arena_run_regind
 arena_run_to_miscelm
 arena_salloc
@@ -381,15 +386,6 @@ pages_map
 pages_purge
 pages_trim
 pages_unmap
-ph_first
-ph_insert
-ph_merge
-ph_merge_aux_list
-ph_merge_ordered
-ph_merge_pairs
-ph_new
-ph_remove_first
-ph_remove
 pow2_ceil_u32
 pow2_ceil_u64
 pow2_ceil_zu