add paths to datum-order
  closes PR 14225
use built-in symbol<?
fix nested ?? in template form
  closes PR 14170

Instead of checking free pvars of first template and using that to choose
which template to use, just try first and on failure try second.

Note: ?? still only covers absent pvars; non-#f non-stx value still
causes error; ellipsis rep mismatch still causes error; etc.