Add `utils/noop`.

Add `utils/noop`, a function which does nothing whatsoever.
Change a call of `utils/idem` to `utils/noop`.

Change a call of `utils/idem` where the return values were ignored to
call `utils/noop` instead.
Enable `string/eval` to handle runtime errors.

Enable `string/eval` to customizably handle runtime errors in the given
string as well as, and separately from, compile-time errors in that
string.