Re: Parameterise benchmarks

[Chris Warburton]

asv-nix has been modified so that:

 - The project being benchmarked is now checked out from git and
   incorporated into the building of the benchmark environment.
 - Benchmark environments are built first using nix-build, then the
   benchmarks are invoked using that environment's bin/python. This
   should be *much* faster than calling nix-shell for every run.
 - Benchmark environments are now split into three parts:
  - Each dependency has a name, a "builder" and a list of "versions":
   - The "matrix" setting maps each name to a list of versions.
   - The "builders" setting maps each name to a builder.
   - A version is an arbitrary Nix expression, which is given as an
     argument to the builder.
   - A builder is a Nix expression, for a function accepting a version
     and a benchmarks directory.
  - The "installer" is a Nix expression, for a function accepting the
    results of the builders and a directory to be benchmarked.

We need to alter our benchmarks to reflect these changes, but it gives
us exactly the flexibility we needed.