Move test of eval builtin to step6. Fix nim implementation.

Move test of eval builtin to step6. Fix nim implementation.

The test makes more sense in step6.
Thanks to kanaka for the read-string suggestion.

Introduction of tail call optimization in step5 was making let* affect
the parent environment. This was partially fixed in later steps.

Introduction of macros in step8 was breaking the evaluation of an
empty list. This was fixed by step 9 but never backported.