Add hooks for extending arglist lookup

* *valid-operator-symbol-p*: functions to call to determine if a is a
   valid operator in current context

* *arglist-hooks*: functions to call to get the arglist of a specific
   operator, uses the first return value from the first function to
   return true in either of the first 2 values (so an empty arglist can be
   indicated by (values nil t) )
Bind *PACKAGE* in COMPLETIONS so local package nicknames/aliases can be found