[callables] Test kernel call with non 1-step slice (#396)

* test a non1-step slice

* highlight the difference between make_slab and slice's access map

* corrects domain for slices with 'start'

* sharpens error condition

- non-integral step sizes cannot be supported in loopy's domain representation

* tests slices with more initial start expressions

* make_slab: step -> iname_multiplier, disallow -ve multipliers

- make_slab and slices are different, using the same names is confusion. Besides
that, 'step' wasn't a representative name for the input