tensors.tensor-slice: make step-slice not extend slice.

tensors.tensor-slice: make step-slice not extend slice.

This caused a small regression in compiler.tree.cleanup on this test:

{ t } [
    [ { array } declare 2 <groups> [ . . ] assoc-each ]
    \ nth-unsafe inlined?
] unit-test

I'm not entirely sure why it wasn't able to infer the slice that was created
for iteration stays a slice, and never becomes a step-slice, so perhaps there
is some improvement to be made in type inference here.