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/src/raw_vec.rs

@@ -170,7 +170,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
         Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
     }
 
-    fn allocate_in(capacity: usize, init: AllocInit, mut alloc: A) -> Self {
+    fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
         if mem::size_of::<T>() == 0 {
             Self::new_in(alloc)
         } else {