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(,).

Instead of is_cr_convertible.

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().

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.

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

The bool conversion happens after predicate is called automatically.

Allows for eg, checking all values equal 2: hb_all (it, 2).

They do absolutely same thing.  hb_requires is to encode constraints,
whereas hb_enable_if is for more conditional enabling.

Part of https://github.com/harfbuzz/harfbuzz/issues/1648