Determines the first item type of a tuple or dictionary entry type.
This function may only be used with tuple or dictionary entry types, but must not be used with the generic tuple type G_VARIANT_TYPE_TUPLE.
In the case of a dictionary entry type, this returns the type of the key.
NULL is returned in case of type being G_VARIANT_TYPE_UNIT.
This call, together with Variant.typeNext provides an iterator interface over tuple and dictionary entry types.