iface: export a few more functions from BinIface

The newly exported functions, under an "internal" haddock section, are the only ones needed to write variants of {get,put}WithUserData (which don't impose Binary nor the IO monad, but instead take BinHandle -> a -> m () and BinHandle -> m a functions respectively -- I need this because I need to access some RnM context while reading/writing my values).