use sb-md5 if on sbcl, otherwise improve error if missing md5sum

[bo-tato]

Currently it is computing md5sum by running the cli program with uiop:

(uiop:run-program (format nil "md5sum \"~A\" | cut -d' ' -f 1" file)
                    :output '(:string :stripped t))

Unfortunately in shells without pipefail set, if cut is present but md5sum is missing this will not return a bad error status, so uiop won't signal an error, leading to a md5-mismatch error later without a clear reason why, as in #8

This changes it to use sb-md5 if we are on sbcl, and if not on sbcl then still shell out to md5sum, but without piping to cut so that uiop will signal a clear error: Couldn't execute "md5sum": No such file or directory

[rudolfochrist]

Great. Thank you for looking into this. Maybe we should add md5sum to the PREREQUISITES section in the README.

[bo-tato]

It looks like we can use openssl: openssl dgst -md5 FILENAME, which is installed by default on mac from what I can find with internet searching, I don't have a mac to test. I imagine openssl will be installed on all machines where curl with https is working