`Hook::bind_async` and `async` support in the `bind!` macro

[maciejhirsz]

The CSV editor example showcases the new async support in the bind! macro:

https://github.com/maciejhirsz/kobold/blob/0a722429837224ff4bc1d95aa9dda40468c723f9/examples/csv_editor/src/main.rs#L12-L37

Will likely need to turn it into a proc macro soon for error handling unfortunately.