Vectorized pipelines with individual result types

At the moment, we require all results of a vectorized pipeline to have the same data type and value type. We should relax this requirement to allow each result to be represented in the most suitable way.

One approach to achieve this is to refactor `MTWrapper` and all related components for the superclass `Structure` (or at least `Matrix`, since we don't support `Frame`s in the vectorized engine yet) as the output type. Special behavior required for a particular data type could be implemented by the data type itself (and be accessible via the `Structure`/`Matrix` interface). This would also facilitate extensibility with new data types.

Start by tracing the vectorized engine's use of MTWrapper and the Structure and Matrix interfaces; the issue notes that Frame is not currently supported. Determine how result data types are enforced today and define what must change so each result can use a suitable representation while preserving extensibility for new data types.

Written by the indexing model from the issue text.