Both to save ops, and also because lambdas don't implement operator!=,
so this was failing in range-based for loop if a lambda was passed to
hb_map() or hb_filter().  Just check end-condition assuming that we
are comparing to .end() or iterators that are otherwise derived from
current iterator.  Ie. don't compare things that are expected to be
in common.

See comments.

This make output of hb_enumerate() sorted regardless of input iterator.

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).