Fix new multidrop bug (shallow copy getting out of sync with the original)
Get rid of inventory::remove_item(int index)
Eliminate inventory::remove_item_by_quantity
Eliminate (unused) inventory::remove_item(int, int)
Convert inventory::remove_stack(int, int) to use invlet
Convert inventory::remove_stack(int) to use invlets
Remove inventory::index_by_letter

Sort of a hack in two places, but those functions should be rewritten *anyway*.

Only one more inventory method that uses index! Woo-hoo!
Remove inventory::operator[]. No significant old indexed functions!

Well, that was a doozy. Finally got inventory::operator[] excised. Some stuff is still using slices (in particular, UI and AI), and those should probably be moved into friend classes or something at some point.

Now time for testing.