Rollup merge of #76993 - blitzerr:alloc-ref, r=Amanieu

Changing the alloc() to accept &self instead of &mut self

Fixes: [#55](https://github.com/rust-lang/wg-allocators/issues/55)

This is the first cut. It only makes the change for `alloc` method.

library/alloc/tests/heap.rs

@@ -11,7 +11,7 @@ fn std_heap_overaligned_request() {
     check_overalign_requests(Global)
 }
 
-fn check_overalign_requests<T: AllocRef>(mut allocator: T) {
+fn check_overalign_requests<T: AllocRef>(allocator: T) {
     for &align in &[4, 8, 16, 32] {
         // less than and bigger than `MIN_ALIGN`
         for &size in &[align / 2, align - 1] {