syntax_ext: Improve and simplify code generated by #[global_allocator]

Instead of
```
mod allocator_abi { /* methods */ }
```
we now generate
```
const _: () = { /* methods */ }
```
and use `std_path` for paths referring to standard library entities.

This way we no longer need to generate `use` and `extern crate` imports, and `#[global_allocator]` starts working inside unnamed blocks.

src/test/run-pass/allocator/custom-in-block.rs

@@ -13,9 +13,10 @@ fn main() {
     #[global_allocator]
     pub static GLOBAL: A = A(AtomicUsize::new(0));
 
+    let n = GLOBAL.0.load(Ordering::SeqCst);
     let s = Box::new(0);
     helper::work_with(&s);
-    assert_eq!(GLOBAL.0.load(Ordering::SeqCst), 1);
+    assert_eq!(GLOBAL.0.load(Ordering::SeqCst), n + 1);
     drop(s);
-    assert_eq!(GLOBAL.0.load(Ordering::SeqCst), 2);
+    assert_eq!(GLOBAL.0.load(Ordering::SeqCst), n + 2);
 }

src/test/run-pass/allocator/custom-in-submodule.rs

@@ -17,9 +17,10 @@ mod submodule {
 }
 
 fn main() {
+    let n = submodule::GLOBAL.0.load(Ordering::SeqCst);
     let s = Box::new(0);
     helper::work_with(&s);
-    assert_eq!(submodule::GLOBAL.0.load(Ordering::SeqCst), 1);
+    assert_eq!(submodule::GLOBAL.0.load(Ordering::SeqCst), n + 1);
     drop(s);
-    assert_eq!(submodule::GLOBAL.0.load(Ordering::SeqCst), 2);
+    assert_eq!(submodule::GLOBAL.0.load(Ordering::SeqCst), n + 2);
 }