Add imenu support for 'aio-defun'

[kaiwk]

it works after (require 'aio) and (revert-buffer)

[skeeto]

Thanks! I'd like to accept this change, but I have two small requests:

• IMHO, putting these under an "Async" grouping is not appropriate. Asynchronous functions are installed into the same namespace as normal, synchronous functions. It's not a new type of thing, just a behavior.

• I'd like the hook function to have a name, even if it's just a "private" (contains --) name. That way if someone looks at their imenu-generic-expression they won't just see some closure. It also makes it easier to remove.