Use more assert_{future,stream} functions

[stepancheg]

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.

[taiki-e]

Thanks