In the nightly it appears that `no_mangle` is now considered "unsafe",
so we need to allow that in the ffi modules. For the layout code this
patch also creates that ffi module with prefixed function names, like in
the other modules and only allows unsafe in there.
Don't require the caller to Box the closure. With the assumption that
the majority of callers *want* the closure to be boxed (i.e. it's not
already boxed), the API becomes easier to use.
