Enhance the `GenerateSliceOfXxxx()`-family to create variable length

[mwsis]

Without changing parameters, can add ability to generate variable length slices. To achieve this, need to add following:

• the generate functions' size parameter can be negative; and
• we add a public custom error - e.g. EndOfGeneration - that can be returned by the generator function(s) and will cause successful termination of generation;