Exportable macros require hardcoding crate's name

which means the user has to import it under that name to use the macros.

``` rust
// some_crate.rs
#[feature(macro_rules)];

pub static X: uint = 1;

#[macro_export]
macro_rules! foo { () => { some_crate::X } }
```

``` rust
// other_crate.rs
#[feature(phases)];
#[phase(syntax, link)] extern mod blah = "some_crate";

fn main() { foo!(); } // unresolved name some_crate
```
