Stabilize a portion of 'once_cell'

Move items not part of this stabilization to 'lazy_cell' or 'once_cell_try'

src/librustdoc/lib.rs

@@ -11,7 +11,7 @@
 #![feature(let_chains)]
 #![feature(test)]
 #![feature(never_type)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(type_ascription)]
 #![feature(iter_intersperse)]
 #![feature(type_alias_impl_trait)]