Fix double referencing of SIDE_TYPE_OPTIONAL

Fix double referencing of SIDE_TYPE_OPTIONAL

An optional type was a pointer to a `struct side_type'.  This results in
having OPTIONAL -> OPTIONAL -> TYPE when defining an optional.

Fix this by defining a `struct side_type_optional' that has a pointer to
a `struct side_type', just like other containers.

Change-Id: I4100fad7485d309e4f3ba798227eac0694fcc294
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>