Rollup merge of #95579 - Cyborus04:slice_flatten, r=scottmcm

Add `<[[T; N]]>::flatten{_mut}`

Adds `flatten` to convert `&[[T; N]]` to `&[T]` (and `flatten_mut` for `&mut [[T; N]]` to `&mut [T]`)

library/alloc/src/lib.rs

@@ -131,6 +131,7 @@
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
 #![feature(try_trait_v2)]
+#![feature(unchecked_math)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
 //