Move small run metadata into the arena chunk header.

Move small run metadata into the arena chunk header, with multiple
expected benefits:
- Lower run fragmentation due to reduced run sizes; runs are more likely
  to completely drain when there are fewer total regions.
- Improved cache behavior.  Prior to this change, run headers were
  always page-aligned, which put extra pressure on some CPU cache sets.
  The degree to which this was a problem was hardware dependent, but it
  likely hurt some even for the most advanced modern hardware.
- Buffer overruns/underruns are less likely to corrupt allocator
  metadata.
- Size classes between 4 KiB and 16 KiB become reasonable to support
  without any special handling, and the runs are small enough that dirty
  unused pages aren't a significant concern.

include/jemalloc/internal/private_symbols.txt

@@ -42,6 +42,8 @@ arena_mapbitsp_read
 arena_mapbitsp_write
 arena_maxclass
 arena_miscelm_get
+arena_miscelm_to_pageind
+arena_miscelm_to_rpages
 arena_new
 arena_palloc
 arena_postfork_child
@@ -61,6 +63,7 @@ arena_ralloc_junk_large
 arena_ralloc_no_move
 arena_redzone_corruption
 arena_run_regind
+arena_run_to_miscelm
 arena_salloc
 arena_sdalloc
 arena_stats_merge