integrate Lazy into std layout

This commit refactors the initial implementation to fit into std and
makes some other changes:

- use MaybeUninit internally in SyncOnceCell
- correctly impl Drop for lazy::Once
- port Lazy::take from once_cell from: https://github.com/matklad/once_cell/pull/100

Co-Authored-By: Paul Dicker <pitdicker@users.noreply.github.com>

src/libstd/lib.rs

@@ -285,6 +285,7 @@
 #![feature(linkage)]
 #![feature(llvm_asm)]
 #![feature(log_syntax)]
+#![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_ref)]
 #![feature(maybe_uninit_slice)]
 #![feature(min_specialization)]
@@ -292,6 +293,7 @@
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(nll)]
+#![feature(once_cell)]
 #![feature(optin_builtin_traits)]
 #![feature(or_patterns)]
 #![feature(panic_info_message)]
@@ -476,10 +478,7 @@ pub mod process;
 pub mod sync;
 pub mod time;
 
-#[unstable(
-    feature = "std_lazy",
-    issue = "99",
-)]
+#[unstable(feature = "once_cell", issue = "68198")]
 pub mod lazy;
 
 #[stable(feature = "futures_api", since = "1.36.0")]