bug in check-formal

[bamboospirit]

Hello, first I want to thank you for developing this very cool lib. I find it very useful.

In order to integrate checkl with fiveam I had to make two simple changes in the code:
In formalize.lisp I changed

do (5am:is (result-equalp last-result result)))))```
to
```do (5am:is (result-equalp prev val)))))```
and then I wrapped check-formal with a (eval-when (:compile-toplevel :load-toplevel :execute)