[subset] Remove subset-glyf; want everything to point to new iter-based edition. Some of the code will resurface as impl builds out.

hb_identity returns rvalue-reference if input is rvalue.  That, can leak
the reference and cause in bad access to temporaries after they are
destructed.  This is unfortunately unfixable given the desired
transparency of hb_identity :(.  Just don't use it with hb_map().

Older compilers don't like calling iter() from return-type decltype()

../src/hb-map.hh:226:12: error: cannot call member function 'decltype ((((+ hb_array(((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::items, (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask ? (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask + 1) : 0))) | hb_filter((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: is_real))) | hb_map((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: get_pair)))) hb_hashmap_t<K, V, kINVALID, vINVALID>::iter() const [with K = const hb_serialize_context_t::object_t*; V = unsigned int; K kINVALID = 0u; V vINVALID = 0u; decltype ((((+ hb_array(((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::items, (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask ? (((const hb_hashmap_t<K, V, kINVALID, vINVALID>*)this)->hb_hashmap_t<K, V, kINVALID, vINVALID>::mask + 1) : 0))) | hb_filter((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: is_real))) | hb_map((& hb_hashmap_t<K, V, kINVALID, vINVALID>::item_t:: get_pair)))) = hb_map_iter_t<hb_filter_iter_t<hb_array_t<hb_hashmap_t<const hb_serialize_context_t::object_t*, unsigned int, 0u, 0u>::item_t>, bool (hb_hashmap_t<const hb_serialize_context_t::object_t*, unsigned int, 0u, 0u>::item_t::*)() const, const<anonymous struct>&, 0u>, hb_pair_t<const hb_serialize_context_t::object_t*, unsigned int> (hb_hashmap_t<const hb_serialize_context_t::object_t*, unsigned int, 0u, 0u>::item_t::*)() const, 0u>]' without object
     + iter()
            ^
../src/hb-meta.hh:58:41: note: in definition of macro 'HB_AUTO_RETURN'
 #define HB_AUTO_RETURN(E) -> decltype ((E)) { return (E); }
                                         ^

As 503748d8 fix

also snake_cased popSubrtNum and other surrounding function names

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=14641

The iter objects shouldn't not be const.  D'oh.