Rollup merge of #57825 - RalfJung:zeroed, r=nikomatsakis
Rollup merge of #57825 - RalfJung:zeroed, r=nikomatsakis

un-deprecate mem::zeroed

as per the discussion around <https://github.com/rust-lang/rust/issues/53491#issuecomment-451454793>

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust