[tracking] Streams parity

Similar to https://github.com/async-rs/async-std/issues/129, this tracks all missing methods and functions for `ParallelStream`. Each method and function should take an async closure as an argument (even if it's still slightly cumbersome to do so), and expose its return future.

If you'd like to implement a method or a function, just comment on this issue and it's yours!

## Missing free functions
- [ ] `from_fn`
- [ ] `repeat_with`
- [ ] `successors`

## Missing traits
- [ ] `ParallelExtend`
- [ ] `ParallelProduct`
- [ ] `ParallelSum`

## Missing stream methods
- [ ] `ParallelStream::all`
- [ ] `ParallelStream::any`
- [ ] `ParallelStream::by_ref`
- [ ] `ParallelStream::chain`
- [ ] `ParallelStream::cloned`
- [ ] `ParallelStream::cmp`
- [ ] `ParallelStream::collect`
- [ ] `ParallelStream::copied`
- [ ] `ParallelStream::count`
- [ ] `ParallelStream::cycle`
- [ ] `ParallelStream::enumerate`
- [ ] `ParallelStream::eq`
- [ ] `ParallelStream::filter`
- [ ] `ParallelStream::filter_map`
- [ ] `ParallelStream::find`
- [ ] `ParallelStream::find_map`
- [ ] `ParallelStream::flat_map`
- [ ] `ParallelStream::flatten`
- [ ] `ParallelStream::fold`
- [x] `ParallelStream::for_each`
- [ ] `ParallelStream::fuse`
- [ ] `ParallelStream::ge`
- [ ] `ParallelStream::gt`
- [ ] `ParallelStream::inspect`
- [ ] `ParallelStream::last`
- [ ] `ParallelStream::le`
- [ ] `ParallelStream::lt`
- [x] `ParallelStream::map`
- [ ] `ParallelStream::max`
- [ ] `ParallelStream::max_by`
- [ ] `ParallelStream::max_by_key`
- [ ] `ParallelStream::min`
- [ ] `ParallelStream::min_by`
- [ ] `ParallelStream::min_by_key`
- [ ] `ParallelStream::ne`
- [x] `ParallelStream::next`
- [ ] `ParallelStream::nth`
- [ ] `ParallelStream::partial_cmp`
- [ ] `ParallelStream::partition`
- [ ] `ParallelStream::peekable`
- [ ] `ParallelStream::position`
- [ ] `ParallelStream::product`
- [ ] `ParallelStream::rev`
- [ ] `ParallelStream::rposition`
- [ ] `ParallelStream::scan`
- [ ] `ParallelStream::size_hint`
- [ ] `ParallelStream::skip`
- [ ] `ParallelStream::skip_while`
- [ ] `ParallelStream::step_by`
- [ ] `ParallelStream::sum`
- [ ] `ParallelStream::take`
- [ ] `ParallelStream::take_while`
- [ ] `ParallelStream::try_fold`
- [ ] `ParallelStream::try_for_each`
- [ ] `ParallelStream::unzip`
- [ ] `ParallelStream::zip`

## Missing `FromStream` impls
- [ ] `FromParallelStream<()> for ()`
- [ ] `FromParallelStream for String`
- [ ] `FromParallelStream for String`
- [ ] `FromParallelStream<&'a char> for String`
- [ ] `FromParallelStream<&'a str> for String`
- [ ] `FromParallelStream for Cow<'a, [T]> where T: Clone`
- [ ] `FromParallelStream for Box<[A]>`
- [ ] `FromParallelStream for VecDeque`
- [ ] `FromParallelStream> for Result where V: FromStream`
- [ ] `FromParallelStream> for Option where V: FromStream`
- [ ] `FromParallelStream<(K, V)> for BTreeMap where K: Ord`
- [ ] `FromParallelStream<(K, V)> for HashMap where K: Eq + Hash, S: BuildHasher + Default`
- [ ] `FromParallelStream for BinaryHeap where T: Ord`
- [ ] `FromParallelStream for BTreeSet where T: Ord`
- [ ] `FromParallelStream for LinkedList`
- [ ] `FromParallelStream for Vec`
- [ ] `FromParallelStream for HashSet where T: Eq + Hash, S: BuildHasher + Default`