Generalised Fold to Row/Column

[ColmBhandal]

Requirement

The 2D array extension has functions to fold to a single column/row but the target type of the fold is the same as the array type. In general, the target type could be different- this is the standard case for fold operations. For this feature, we want more general version of the fold to single row/column functions that allow the target type to be different from the source type.

Value Proposition

This solves a more general use case for fold than does the existing functions whose target type matches the source type.

Design Ideas

The original fold functions can be refactored in terms of the more general ones.