Info: do not print empty dispatches: they are equivalent to idtac.
Fixup.
Fixup.
Info: placeholder for the tactics which are still in old style in tacinterp.
Fixup: idtac printing.
Info: print correctly Ltac-level aliases.

Previously: would keep only one of a class of aliases.
Info: print correctly Ltac definitions which are not syntactically functions or tactics.

Previous version lost the name for [Ltac t := let … in …], or [match], etc…