feat: support default implement method

[Sherlock-Holo]

that should fix #17

[tmandry]

Thanks for your PR. Unfortunately I think it is too simplistic to land. In order to capture references to Self we will need where Self: Sync bounds (at least in the case of a Send trait.. in the general case, I think the type checker may not be up to the task, so we will have to keep it limited to Send for now). We also need to make sure arguments are moved into the async block, as is done in #20.