FEAT: nodrop: No odds dependency

Copy the required unreachable and debug assertion; saves us dependency
on odds.

We could also depend on crate unreachable, but that adds dep on
2 microcrates.

nodrop/src/lib.rs

@@ -19,9 +19,6 @@
 #[cfg(not(any(test, feature="std")))]
 extern crate core as std;
 
-#[cfg(not(feature = "use_union"))]
-extern crate odds;
-
 #[cfg(feature = "use_union")]
 extern crate nodrop_union as imp;
 
@@ -30,7 +27,6 @@ pub use imp::NoDrop;
 
 #[cfg(not(feature = "use_union"))]
 mod imp {
-    use odds::debug_assert_unreachable;
     use std::ptr;
     use std::mem;
     use std::ops::{Deref, DerefMut};
@@ -125,6 +121,14 @@ mod imp {
         assert!(mem::size_of::<Flag<&i32>>() > mem::size_of::<&i32>());
         assert!(mem::size_of::<Flag<Vec<i32>>>() > mem::size_of::<Vec<i32>>());
     }
+
+    // copying this code saves us microcrate deps
+    #[inline]
+    unsafe fn debug_assert_unreachable() -> ! {
+        debug_assert!(false, "Reached unreachable section: this is a bug!");
+        enum Void { }
+        match *(1 as *const Void) { }
+    }
 }
 
 #[cfg(test)]