Was turned into function when we had static ones and wanted to be
move-safe...  Not the case anymore.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=960331

Fixes https://github.com/harfbuzz/harfbuzz/issues/1662

Any way to catch these?

There are two cases that we accept.  Encode both.

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().