* fix: enumerate Promise<array> (e.g. in {% for ... %})
Previously, a Promise of an array was not being enumerated in
{% for %}, for example. This is misleading since the library
handles promises elsewhere (e.g. if you {% assign x = promiseArray %}
and then {% for v in x %}, it worked just fine.
This PR makes Promises of arrays handled by changing toEnumerable
to handle then-ables. This affects other iterators, too, e.g. tablerow,
so I put in a test for that as well.
