Add type alias BoxFusedStream

[mzabaluev]

Like BoxStream, this can be used for convenience.
Unlike BoxStream, the type is usable directly in select!.
Also add LocalBoxFusedStream.

[taiki-e]

Thanks for the PR.
I would prefer not to expose more (Local)Boxed* type aliases. If we accept this PR, we will probably also need (Local)BoxedFusedFuture for consistency. And the same can happen if new stream related traits are added.

[mzabaluev]

The case for these aliases is to facilitate direct use of boxed futures/streams in select!, using the fuse logic that is provided by the boxed future through dynamic dispatch.

I would not advocate proliferation of arbitrary boxed trait aliases for the sake of convenience alone.

[taiki-e]

Sorry for the delayed reply.

I'm considering removing Fused* traits in the next major version (#2207) (also it may be deprecated in the current major version).
Also, it is difficult to implement Fused* traits correctly without specialization, so I personally do not recommend relying on those implementations.
So I recommended using Fuse<BoxedFuture<..>> or defining your own type alias, instead of improving convenience for features that will be deprecated.

I'm going to close this, but thanks @mzabaluev for the patch anyway.