Rename unwind.rs to reflect its contents. (#38)

Rename it to `unwind_unimplemented.rs` so that the code in lib.rs is
more self-describing.

Author: sunfishcode

src/lib.rs

@@ -18,7 +18,7 @@ extern crate alloc;
 #[cfg(not(target_arch = "arm"))]
 extern crate unwinding;
 #[cfg(target_arch = "arm")]
-mod unwind;
+mod unwind_unimplemented;
 
 pub mod program;
 #[cfg(feature = "signal")]