Support multi-indexes in IndexDefine

With #163 closed with improved index support, multi-indexes weren't included in that issue, so, here's this one instead.  This requires identifying where multi-indexes behave differently than normal indexes (eg. GetAll).  The index is defined on an array, but queried on individual elements.  A different interface will need to be created and additional methods added to Query to support type-safe multiindex access.

TableTests.IndexCreateMulti should be cloned and used with the new type-safe approach.