Implement pz2ind(), pind2sz(), and psz2u().

These compute size classes and indices similarly to size2index(),
index2size() and s2u(), respectively, but using the subset of size
classes that are multiples of the page size.  Note that pszind_t and
szind_t are not interchangeable.

include/jemalloc/internal/private_symbols.txt

@@ -393,6 +393,7 @@ pages_map
 pages_purge
 pages_trim
 pages_unmap
+pind2sz
 pow2_ceil_u32
 pow2_ceil_u64
 pow2_ceil_zu
@@ -446,6 +447,8 @@ prof_thread_active_init_set
 prof_thread_active_set
 prof_thread_name_get
 prof_thread_name_set
+psz2ind
+psz2u
 purge_mode_names
 register_zone
 rtree_child_read