add some futures_core::stream::Stream impls for AsyncInputStream

[pchickey]

WIP: This PR still needs some tests

Closes #77

[teohhanhui]

Calling into_stream() on an AsyncInputStream feels odd. 😆

Could this instead be:

impl AsyncInputChunkStream {
    pub fn new(stream: AsyncInputStream) -> Self {
        Self {
            stream,
            chunk_size: 8 * 1024,
        }
    }

    pub fn with_size(stream: AsyncInputStream, chunk_size: usize) -> Self {
        Self {
            stream,
            chunk_size,
        }
    }
}

which makes it somewhat analogous to Vec::new and Vec::with_capacity

[pchickey]

I don't think anyone wants to think about AsyncInputChunkStream as a type itself, I would make it a private type and just return impl Stream<Item = ...> since that is all that matters, except we need to return a concrete type in case the user wants the into_inner method on the ChunkStream/ByteStream. I don't think the analogy to Vec really holds much water.