Make the crate no_std (#15)

* Remove crossbeam

* Rewrite notification and registration

* Make crate no_std

* Add a comment
diff --git a/src/lib.rs b/src/lib.rs
index a265679..5fe858a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -97,10 +97,13 @@
 //! [`Task`]: struct.Task.html
 //! [`JoinHandle`]: struct.JoinHandle.html
 
+#![no_std]
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
 #![doc(test(attr(deny(rust_2018_idioms, warnings))))]
 #![doc(test(attr(allow(unused_extern_crates, unused_variables))))]
 
+extern crate alloc;
+
 mod header;
 mod join_handle;
 mod raw;