macros: `tt` fragments treat `$(foo)*` like `foo`

[jseyfried]

For example, the following compiles and prints success:

macro_rules! identity {
    ($t:tt) => { $t }
}

identity! {
    $(fn $(main)* () {
        $($( println!($("success")+); )+)+
    })*
}

[jseyfried]

cc @nrc