Make the crate no_std (#15)

* Remove crossbeam

* Rewrite notification and registration

* Make crate no_std

* Add a comment
diff --git a/src/utils.rs b/src/utils.rs
index 441ead1..7c71deb 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -1,5 +1,5 @@
-use std::alloc::Layout;
-use std::mem;
+use core::alloc::Layout;
+use core::mem;
 
 /// Calls a function and aborts if it panics.
 ///
@@ -10,7 +10,7 @@
 
     impl Drop for Bomb {
         fn drop(&mut self) {
-            std::process::abort();
+            unsafe { libc::abort() }
         }
     }