Those of us who were raised the only child of a single parent know how intense the relationship can be. Different days you may play the role of child, parent, sibling or emotional crutch — some days all at once. As you get through it, you try to find space for yourself, to become someone outside the all-enveloping world that your parent has provided. This is the main struggle driving iO Tillett Wright’s debut memoir, “Darling Days.”