Features:
+ configurable via environment variables
+ can skip the actual launching of the lisp implementation (source it
with NIX_LISP_SKIP_CODE=1 to get all the settings)
+ currently supports SBCL, CLisp, ECL
+ determines lisp implementation from NIX_LISP_COMMAND variable or
from buildInputs
+ sets ASDF search path for packages using buildInputs
