Parameterise benchmarks

[Chris Warburton]

Benchmarks need to work across multiple commits. We should parameterise
them, such that:

 - Environment variables (cached definitions, cached theorems, etc.) are
   built once, from the current source tree, and used by all scripts.
 - Benchmark scripts are built once, from the current source tree, and
   used by all scripts.
 - Racket code used in a benchmarking script can be swapped out, so we
   can compare different commits.

asv's environment mechanism should allow us to do this. Currently, we
have the following setup:

 - Benchmarks are defined in asv/benchmarks.py. These run programs.
 - Those programs are defined in asv/benchmarks.nix. They set env vars
   and hard-code file paths in scripts/

We should alter this so that scripts/ is looked up dynamically. The
install/uninstall methods of NixEnvironment should be altered to allow
this.