Update various forward declarations to use typedef

Update various forward declarations to use typedef

There are a number of forward declarations that use struct but not the
customary typedef, because that could have led to repeat typedefs,
which was not allowed.  This is now allowed in C11, so we can update
these to provide the typedefs as well, so that the later uses of the
types look more consistent.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/10d32190-f31b-40a5-b177-11db55597355@eisentraut.org