Request: The ability for a map to iterate over a list

[dqnykamp]

We should be able to a put a <mathlist>, <numberlist>, <textlist>, <booleanlist> into the <sources> of a <map>, and it should iterate over the items in the list.

[dqnykamp]

This would be a lot easier once we have a strongly typed system. Then, this could be accomplished via sugar, where <sources> could wrap list children with an appropriate <extract>. At this point, I'm inclined to wait on that. Otherwise, we'd need to make significant changes to <map> logic. (Map currently relies on having separate children for each item, which we may not have for a <mathlist> with mergeMathLists).