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This series has also been squeezed into D47633 to provide an overall view.

Commit message:
Have a static constant array mapping numerical ID types to their
canonical representations ('id_type_to_str').

New parse_id_type() that parses a type thanks to 'id_type_to_str' and
with a special case to accept also 'any'.

Have parse_rule_element() use parse_id_type(). A later commit will add
a second call to the latter for the destination ID.