aio: custom operations

[Cloudef]

It might be possible to provide custom operations from outside, and for aio itself to implement backend agnostic complex operations as well.

In theory all this needs is a struct that provides aio.EventSource and a method to check if it's complete or not. In theory this could be used to implement http_fetch and such, the custom op itself could use aio.Dynamic to implement it asynchronously.

[Cloudef]

I feel like there should be stage1 aio, and then stage2 aio, where stage2 uses stage1 to implement more complex ops.