Make `run` into `MyNote::run` marked with `#[entrypoint]` attribute macro

[greenhat]

From #787 (comment) the ergonomics we're looking for should look like:

#[note]
struct MyNote {
    recipient: AccountId,
}

#[note]
impl MyNote {

    #[entrypoint]
    pub fn run(&self, account: &BasicWallet, _args: Word) {
        assert_eq!(self.recipient, account.id());

        ...
    }   
}