Switch from hashmap_core to hashbrown.

As suggested
[here](https://github.com/Amanieu/hashmap_core/pull/10#issuecomment-455866083).

This also allows us to re-enable the basic compile fuzz target.

lib/runtime/src/lib.rs

@@ -32,7 +32,7 @@ extern crate alloc as std;
 extern crate std;
 
 #[cfg(not(feature = "std"))]
-use hashmap_core::{map as hash_map, HashMap};
+use hashbrown::{map as hash_map, HashMap};
 #[cfg(feature = "std")]
 use std::collections::{hash_map, HashMap};