Iters

Changes

- Functions taking `Predicate[T]` have been updated to accept `OptionalPredicate[T]`.
Passing `None` as an argument is identical to passing `bool`.

There are three functions which do not accept `None`, though:
- `drop_while`
- `skip_while`
- `take_while`

This choice is motivated by the fact that it does not make much sense to `do_while(None)`.

Changes

- The following functions have been changed:
- `async_iter` is now an alias of `AsyncIter`;
- `iter` is now an alias of `Iter`;
- `reversed` is now an alias of `iter.reversed`.

Changes

- Changed functions of various arity returning `Awaitable[T]` to async functions returning `T`.
([15](https://github.com/nekitdev/iters/pull/15))

Changes

- Added `await async_iter`, equivalent to `await async_iter.list()`.

Initial release.