Implement metadata statistics.

There are three categories of metadata:

- Base allocations are used for bootstrap-sensitive internal allocator
  data structures.
- Arena chunk headers comprise pages which track the states of the
  non-metadata pages.
- Internal allocations differ from application-originated allocations
  in that they are for internal use, and that they are omitted from heap
  profiles.

The metadata statistics comprise the metadata categories as follows:

- stats.metadata: All metadata -- base + arena chunk headers + internal
  allocations.
- stats.arenas.<i>.metadata.mapped: Arena chunk headers.
- stats.arenas.<i>.metadata.allocated: Internal allocations.  This is
  reported separately from the other metadata statistics because it
  overlaps with the allocated and active statistics, whereas the other
  metadata statistics do not.

Base allocations are not reported separately, though their magnitude can
be computed by subtracting the arena-specific metadata.

This resolves #163.

include/jemalloc/internal/private_symbols.txt

@@ -1,6 +1,7 @@
 a0dalloc
 a0get
 a0malloc
+arena_aalloc
 arena_get
 arena_get_hard
 arena_alloc_junk_small
@@ -50,6 +51,9 @@ arena_mapbitsp_read
 arena_mapbitsp_write
 arena_maxclass
 arena_maxrun
+arena_metadata_allocated_add
+arena_metadata_allocated_get
+arena_metadata_allocated_sub
 arena_migrate
 arena_miscelm_get
 arena_miscelm_to_pageind
@@ -90,6 +94,7 @@ atomic_sub_uint32
 atomic_sub_uint64
 atomic_sub_z
 base_alloc
+base_allocated_get
 base_boot
 base_calloc
 base_node_alloc
@@ -205,6 +210,7 @@ hash_rotl_64
 hash_x64_128
 hash_x86_128
 hash_x86_32
+huge_aalloc
 huge_allocated
 huge_boot
 huge_dalloc
@@ -221,10 +227,13 @@ huge_prof_tctx_set
 huge_ralloc
 huge_ralloc_no_move
 huge_salloc
+iaalloc
+iallocztm
 icalloc
 icalloct
 idalloc
 idalloct
+idalloctm
 imalloc
 imalloct
 in_valgrind
@@ -234,6 +243,7 @@ index2size_lookup
 index2size_tab
 ipalloc
 ipalloct
+ipallocztm
 iqalloc
 iralloc
 iralloct