Wrap result from slicing in new type

[hellemo]

Ideas:

• Type stability
• Repeated indexing into result from slicing
• Specialized sum (to avoid allocations from +(?)
• Index intersection for improved performance

[Zetison]

It would be nice if slicing of a IndexedVarArray resulted in a new IndexedVarArray of reduced dimensionality (instead of a Dictionary). It would also be nice to have vector indexing functionality.