Use `assert_future` and `assert_stream` annotations in almost all
`FutureExt`, `TryFutureExt`, `StreamExt` and `TryStreamExt` trait
fns.

My own style of working with third-party code is doing Cmd-Click
in Idea and look at signature/implementation.

I found these existing `assert_` annotations very helpful in
understanding what this particular function does, what it expects,
what it returns, much faster than reading documentation or opening
actual future/stream implementation.

For example, there's `TryStreamExt::and_then` function:

```
    fn and_then<Fut, F>(self, f: F) -> AndThen<Self, Fut, F>
    where
        F: FnMut(Self::Ok) -> Fut,
        Fut: TryFuture<Error = Self::Error>,
        Self: Sized,
    { ... }
```

What does it do? Does it return a future or a stream?  Is stream
item original stream item or returned from future?  Is error
converted? Is it even `Try` object returned from this function?

Now with the function body:

```
        assert_stream::<Result<Fut::Ok, Fut::Error>, _>(...)
```

it's easier to infer that function returns a stream with the same
items as provided future.

So this diff extends `assert_` function to almost all functions of
these four traits.