Rollup merge of #59451 - TimDiekmann:patch-1, r=sfackler

Add `Default` to `std::alloc::System`

`System` is a unit struct, thus, it can be constructed without any additional information. Therefore `Default` is a noop. However, in generic code, a `T: Default` may happen as in

```rust
#[derive(Default)]
struct Foo<A> {
    allocator: A
}
```

Does this need a feature gate?
Should I also add `PartialEq/Eq/PartialOrd/Ord/Hash`?

src/libstd/alloc.rs

@@ -130,7 +130,7 @@ pub use alloc_crate::alloc::*;
 /// program opts in to using jemalloc as the global allocator, `System` will
 /// still allocate memory using `malloc` and `HeapAlloc`.
 #[stable(feature = "alloc_system_type", since = "1.28.0")]
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Default, Copy, Clone)]
 pub struct System;
 
 // The Alloc impl just forwards to the GlobalAlloc impl, which is in `std::sys::*::alloc`.