Move location so macro can be used in more places

Rust only allows macros to be used strictly after they have been
defined. It's weird, and not needed for this PR, but it will avoid later
developers running into the issues I did while developing this.

src/rt/mod.rs

@@ -1,3 +1,7 @@
+#[macro_use]
+mod location;
+pub(crate) use self::location::Location;
+
 mod access;
 use self::access::Access;
 
@@ -10,10 +14,6 @@ pub(crate) use self::arc::Arc;
 mod atomic;
 pub(crate) use self::atomic::{fence, Atomic};
 
-#[macro_use]
-mod location;
-pub(crate) use self::location::Location;
-
 mod cell;
 pub(crate) use self::cell::Cell;