Use NonNull<Void> instead of *mut u8 in the Alloc trait

Fixes #49608

src/liballoc/lib.rs

@@ -99,6 +99,7 @@
 #![feature(lang_items)]
 #![feature(libc)]
 #![feature(needs_allocator)]
+#![feature(nonnull_cast)]
 #![feature(nonzero)]
 #![feature(optin_builtin_traits)]
 #![feature(pattern)]