Auto merge of #49481 - SimonSapin:core-heap, r=alexcrichton

Move the alloc::allocator module to core::heap

This is the `Alloc` trait and its dependencies.

src/libstd/heap.rs

@@ -12,8 +12,9 @@
 
 #![unstable(issue = "32838", feature = "allocator_api")]
 
-pub use alloc::heap::{Heap, Alloc, Layout, Excess, CannotReallocInPlace, AllocErr};
+pub use alloc::heap::Heap;
 pub use alloc_system::System;
+pub use core::heap::*;
 
 #[cfg(not(test))]
 #[doc(hidden)]