Add test cases for the spread operator

src/spread/mult-empty.case

src/spread/mult-err-expr-throws.case

src/spread/mult-err-iter-get-value.case

src/spread/mult-err-itr-get-call.case

src/spread/mult-err-itr-get-get.case

src/spread/mult-err-itr-step.case

src/spread/mult-err-itr-value.case

src/spread/mult-err-unresolvable.case

src/spread/mult-iter.case

src/spread/sngl-empty.case

src/spread/sngl-err-itr-get-call.case

src/spread/sngl-err-itr-get-get.case

src/spread/sngl-err-itr-get-value.case

src/spread/sngl-err-itr-step.case

src/spread/sngl-err-itr-value.case

src/spread/sngl-err-unresolvable.case