Move hash module from collections to core

src/libcore/lib.rs

@@ -121,6 +121,7 @@ pub mod simd;
 pub mod slice;
 pub mod str;
 pub mod tuple;
+pub mod hash;
 // FIXME #15320: primitive documentation needs top-level modules, this
 // should be `core::tuple::unit`.
 #[path = "tuple/unit.rs"]
@@ -142,4 +143,5 @@ mod std {
     pub use kinds;
     pub use option;
     pub use fmt;
+    pub use hash;
 }