Rollup merge of #81730 - RustyYato:object-safe-allocator, r=Amanieu

Make `Allocator` object-safe

This allows rust-lang/wg-allocators#83: polymorphic allocators

src/test/ui/allocator/object-safe.rs

@@ -0,0 +1,13 @@
+// run-pass
+
+// Check that `Allocator` is object safe, this allows for polymorphic allocators
+
+#![feature(allocator_api)]
+
+use std::alloc::{Allocator, System};
+
+fn ensure_object_safe(_: &dyn Allocator) {}
+
+fn main() {
+    ensure_object_safe(&System);
+}