Revert libcore: unify `gen_<type>` methods on `rand::RngUtil` into the generic `gen`.

This reverts 4a24f10ac6f155699f086051ba6a728d9afb5ca6
Revert "libcore: remove unnecessary deref"

This reverts commit 9860fe10a19cc4997e58861df905f8dbe4de3c5b.
auto merge of #6067 : catamorphism/rust/revert, r=catamorphism

See #6061. Reverting these commits fixes it; later we'll isolate the problem.