Move =? cond to function to reduce generated code

Related to https://github.com/NoahTheDuke/lazytest/issues/13, this moves the big `cond` block from `=?` to a separate function, which reduces generated code size. I also changed the msg generation to be eager so `(pr-str e')` and `(pr-str (list 'expect e' a))` don't bloat the generated code either.