Support chained stream

[ethe]

Got compiling error on below code:

pub fn chained_stream() -> impl Stream {
    #[stream]
    async {
        yield 1;
    }
    .map(|d| d * 2)
}

error: unexpected token
   --> tests/stream.rs:215:6
    |
215 |     .map(|d| d * 2)
    |      ^^^

I think relying on feature stmt_expr_attributes would makes a little bit hard to support this.

[taiki-e]

relying on feature stmt_expr_attributes

FYI, you can use stream_block macro to work around stmt_expr_attributes.