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(,).