This PR provides an iterator combinator that lifts the emitted values
into a higher universe level via `ULift`. This combinator is then used
to make the subarray iterators universe-polymorphic. Previously, they
were only available for `Subarray α` if `α : Type`.
This PR introduces polymorphic slices in their most basic form. They
come with a notation similar to the new range notation. `Subarray` is
now also a slice and can produce an iterator now. It is intended to
migrate more operations of `Subarray` to the `Slice` wrapper type to
make them available for slices of other types, too.
The PR also moves the `filterMap` combinators into `Init` because they
are used internally to implement iterators on array slices.
