Fixtype of structure declarators [C:6.7.2.1] [C:A.2.2].
This is a product type introduced by fty::defprod.
This corresponds to struct-declarator in the grammar in [C]. This is part of structure declarations, so as discussed in structdecl arguably a better name would be `member declarators'.
To make this definition simpler, we allow an absent declarator and an absent expression, even though this is disallowed in the concrete syntax.