Add "nix copy" command

This replaces nix-push. For example,

  $ nix copy --to file:///tmp/cache -r $(type -p firefox)

copies the closure of firefox to the specified binary cache. And

  $ nix copy --from file:///tmp/cache --to s3://my-cache /nix/store/abcd...

copies between two binary caches.

It will also replace nix-copy-closure, once we have an SSHStore class,
e.g.

  $ nix copy --from ssh://alice@machine /nix/store/abcd...

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