DT::Slice is hard to use correctly

#84 (comment 8743065) highlighted that .first and .second are not good variable names and that placing the total before the current slice in aggregate initialization is confusing.

I proposed defining a struct Slice { int slice, total; } so we can use designated initializers, { .slice=0, .total=1 }. The fix for #84 (closed) will not address the source of the confusion.