WIP: Experiment with memoizing output from expensive external commands

Chris Lambrequested to merge
experiment-with-memoizing-output-from-expensive-external-commands-eg-readelf into master

Experiment with memoizing output from expensive external commands, eg. readelf.

See #93. This, unfortunately, does not result in much, if any, saving in the most common-case of comparing two Debian builds but thought I might throw this out here...

(Please don't merge yet, or perhaps even ever...)

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