Notably, add hb_is_source_of(,) and hb_is_sink_of(,) to replace most
uses of hb_is_iterator_of(,).

NameRecord is not copy-constructible, so should be iterator of
const-reference.

If To is const& then From doesn't need to be &.

Instead of is_cr_convertible.

Just so it's easier to use it in other macros.

Use hb_range() instead.

We should compare-equal to end if either iterator's end reaches,
not if both reach at the same time.  Fixes infinite-loop in test
which was happening after hb_enumerate() switched to using hb_zip().

That's not legal.

Also rewrite it via composition.

hb_range() is like Python range.  hb_iota() has slightly different API.
Ie. it takes a start, instead of end.

Usermanual: object-model chapter

Should fix MSVC.

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); }
                                         ^