Consider deprecating *stream_block! macros in favor of #[stream] on async blocks

[taiki-e]

#41 has postponed the removal of *stream_block! macros because of some concerns.

• format issue:

  Given that rustfmt doesn't format #[stream] async {} on let so nicely, I'm wondering if removing stream_block is good...

  https://github.com/taiki-e/futures-async-stream/blob/ff2916fc7d7db35de99b7989622cbd0f155f2374/tests/stream.rs#L99-L105

  (That said, this is the normal format when using attributes on the expr of let. Also, it can be avoided by wrapping it in a block.)

• fn-like proc-macro at the expression position is stable, but attribute macro is not.