syntax: remove ast::Sigil.
rustdoc: fix fallout from removing ast::Sigil.
rustc: fix fallout from removing ast::Sigil and use ty::TraitStore in ty::ClosureTy.
rustc: remove proc -> once || coercions.
rustc: remove the last mentions of @fn.
auto merge of #13461 : eddyb/rust/cleanup-at-fn, r=luqmana