Auto merge of #281 - cole-miller:get-each-mut-array-map, r=Amanieu
Auto merge of #281 - cole-miller:get-each-mut-array-map, r=Amanieu

Replace some custom unsafe code with `array::map`

`<[T; N]>::map` was recently stabilized (rust-lang/rust#87174). It's not on any stable or beta yet, but we're only using it here with `cfg(feature = "nightly")` (in the functions `HashMap::get_each_mut` and `HashMap::get_each_key_value_mut`).

rust-lang/hashbrown,rust-lang/hashbrown