Integrate `pre_check()` into `for_checkr()`

[dtkaplan]

Have pre_check() do the step by step evaluation instead of for_checkr(). Right now there's redundancy: both functions are doing it.

Add an argument to for_checkr() with the solution code, so that spell checks can take place within that context.

Add a SOLN_CODE binding along with USER_CODE. That will make it easier to pass any solution code to the new for_checkr().