Implement VectorSlice

VECTOR_SLICE and VectorSlice were missing.  Add an implmenentation;
perhaps not particulary efficient, though.

* basis/vector_slice.sml, basis/__vector_slice.sml: New files.
* basis/array.sml, basis/__array.sml: Implement Array.vector.
* basis.mlp, basis/require_all.sml, images/GNUmake: Mention new files.