Move replace and swap to std::mem. Get rid of std::util

Also move Void to std::any, move drop to std::mem and reexport in
prelude.
auto merge of #11956 : edwardw/rust/issue-7556, r=cmr

Closes #7556.

Also move ``std::util::Void`` to ``std::any::Void``. It makes more sense to me.