This is so that we can put other build artefacts (e.g. executables) in
properly organised subdirectories of build, e.g. build/bin/chez,
build/bin/js, etc.
With the --yaffle flag, you get the old behaviour which is to invoke the
checker for the core theory (and all the tests are updated appropriately
for this).
Most notably, when elaborating deferring argument, if the hole
standing for the argument is still a hole, fill it in directly rather
than going via unification. This prevents some needless evaluation.
