Detect Map mutations

Besides the problem in #54 when binding an empty Map it seems that any additions to a Map instance using `map.set()` are not detected. Are maps supported with virtual repeat? I thought that was part of abstract repeat strategy. The only way I've been able to get anything to show with a map is to go through a value converter that produces an array and even when I do that any changes in the map are not pushed through the value converter.

Start by reading issue #54 and the virtual repeat and abstract repeat strategy behavior around empty Maps. Reproduce additions made with map.set(), then compare direct Map binding with the value-converter-to-array path. Done means Map contents render initially and subsequent mutations propagate through virtual repeat.

Written by the indexing model from the issue text.