fixes #39 and refactors rectify-prefix-list-req-statement

the bugfix is quite strait forward I think.

The refactor greatly simplifies the prefix list type require handling by creating a much more suitable data structure to work on initially -- see `cljr--delete-and-extract-sexp-with-nested-sexps`. Makes the code more readable as well I assume.

I did not squash the commits since they are quite separate things. Let me know if you prefer otherwise.