Performance tracking

[Neil Jerram]

Here's an idea for performance tracking.  Please let me know what you
think.

- I think the main people interested in performance tracking are the
  core Guile developers, so this proposal is primarily for them (aka
  us).

- Because we all have different machines, we need separate tracking.

- So, each developer who wants to track performance over time creates a
  performance-data/<unique-name> file in their tree, and commits it to
  the repository.

- <unique-name> needs to be associated with the particular machine that
  a particular developer uses, over time, to run performance tests.  For
  example, I might create `performance-data/neil-arudy'.

- The file content is created and updated by running `benchmark-guile >
  performance-data/<unique-name>'.

- You don't have to do this every time you commit any change, but we
  need the convention that whenever a performance-data file _is_
  updated, the data in it is generated by the tree of that commit.

- We can write tools to show changes in performance data over time (or
  over commits).  This should be easy, as the output of benchmark-guile
  is Scheme-readable.

Thanks,
        Neil