This PR significantly changes the signature of the `ToIterator` type
class. The obtained iterators' state is no longer dependently typed and
is an `outParam` instead of being bundled inside the class. Among other
benefits, `simp` can now rewrite inside of `Slice.toList` and
`Slice.toArray`. The downside is that we lose flexibility. For example,
the former combinator-based implementation of `Subarray`'s iterators is
no longer feasible because the states are dependently typed. Therefore,
this PR provides a hand-written iterator for `Subarray`, which does not
require a dependently typed state and is faster than the previous one.
Converting a family of dependently typed iterators into a simply typed
one using a `Sigma`-state iterator generates forbiddingly bad code, so
that we do provide such a combinator. This PR adds a benchmark for this
problem.
