Add recommend changes to array

Switch from indexing to zip, and also use `write` on `MaybeUninit`.

Add array_map feature to core/src/lib

Attempt to fix issue of no such feature

Update w/ pickfire's review

This changes a couple of names around, adds another small test of variable size,
and hides the rustdoc #![feature(..)].

Fmt doctest

Add suggestions from lcnr

library/core/src/lib.rs

@@ -145,6 +145,7 @@
 #![feature(abi_unadjusted)]
 #![feature(adx_target_feature)]
 #![feature(maybe_uninit_slice)]
+#![feature(maybe_uninit_extra)]
 #![feature(external_doc)]
 #![feature(associated_type_bounds)]
 #![feature(const_caller_location)]