DST raw pointers - *-pointers are fat pointers

r? @pnkfelix or @nikomatsakis 
Ah, this felt so much like an enum definition that didn't even cross my mind.

I guess we could have a compulsory trailing comma via $(... , )*`. Also, [`vec!`](http://doc.rust-lang.org/master/std/macro.vec!.html) somehow manages to satisfy the compiler to make optional trailing commas work.

Anyway, not important.