Support phantom types inside a method body

[dtolnay]

The poor interaction between method bodies and handling of relative paths in use statements in Rust makes this extremely challenging or (I currently believe) impossible to support. Doesn't hurt to have an issue though:

use ghost::phantom;

fn main() {
    #[phantom]
    struct MyPhantom<T>;
}

error[E0432]: unresolved import `super::MyPhantom`
 --> src/main.rs:5:12
  |
5 |     struct MyPhantom<T>;
  |            ^^^^^^^^^ maybe a missing `extern crate MyPhantom;`?

error[E0432]: unresolved import `self`
 --> src/main.rs:4:5
  |
4 |     #[phantom]
  |     ^^^^^^^^^^ maybe a missing `extern crate __value_MyPhantom;`?