Copy the code from slab_allocator into OSTD and add license header

ostd/src/lib.rs

@@ -19,6 +19,7 @@
 #![feature(ptr_sub_ptr)]
 #![feature(strict_provenance)]
 #![feature(sync_unsafe_cell)]
+#![feature(allocator_api)]
 // The `generic_const_exprs` feature is incomplete however required for the page table
 // const generic implementation. We are using this feature in a conservative manner.
 #![allow(incomplete_features)]