[PATCH] Add 'guix hash'.

[Nikita Karetnikov]

This patch adds a 'guix hash' command.

I'd like it to output non-Base32 too, but I haven't found a way to
convert the output of 'sha256', which is a bytevector, to a string.

What options would you like to see implemented?

Should we capitalize error messages?  The current one was copied from
'guix import'.

Is it OK to output a backtrace when the specified file doesn't exist?

For example:

  # ./pre-inst-env guix hash foobar

Should we only print the error message?

0001-Add-guix-hash.patch
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