Remove Item type

Continuing the theme of cleaning up old and weird stuff in the code, I
propose to remove the Item type. It's defined as

typedef Pointer Item;

(note that Pointer is char *), and its almost only use is as part of
PageAddItem*():

extern OffsetNumber PageAddItemExtended(Page page, Item item, Size size,
OffsetNumber offsetNumber, int flags);

The actual thing passed to PageAddItem*() is of some AM-specific type,
and so that forces casts in all calls, like

offnum = PageAddItem(page, (Item) tuple, tuplen, offnum, true, false);

So this type doesn't promote any type safety, but in fact sort of forces
the opposite.

And then we have the usual problem that this is a type that hides
pointerness, and so we can't add any qualifiers, which leads to some
unconstify nonsense.

(Also, Item is unrelated to ItemPointer, which seems confusing.)

Initially I considered creating some underlying type to point to like
ItemData, but that didn't seem useful. Ultimately, PageAddItem*() is
just a fancy memcpy(), and using void * is an idiomatic way to refer to
a generic chunk of memory. So I'm proposing to remove the Item type,
replace it with void * in function prototypes, and remove all the casts.

Extension authors can make their code backward compatible by replacing

PageAddItem(page, (Item) tuple, ...)

with

PageAddItem(page, (void *) tuple, ...)