In particular, don't use base-64, which we don't support. (We do have
base-32 redirects for hysterical reasons.)
Also, add a test for the hashed mirror feature.
This removes the need to have multiple downloads in the stdenv
bootstrap process (like a separate busybox binary for Linux, or
curl/mkdir/sh/bzip2 for Darwin). Now all those files can be combined
into a single NAR.