Function: number_of_items

--Usage--
> number_of_items(in: some_collection)

Returns the position of an item in a list or characters in a string.

--Parameters--
! Parameter	Type				Description
| @in@		[[type:list]] or [[type:map]] or [[type:set]] or [[type:string]]
		  				Object to determine the number of items of.
		  				When it is a set, returns the number of cards.
		  				When it is a string, returns the number of characters.
| @filter@	[[type:function]] (optional)	Act as if the list was [[fun:filter_list|filtered]] by this criterion.
		  				Only when @in@ is a set.

--Examples--
> number_of_items(in: ["x", "y", "z"]) == 3
> number_of_items(in: "pink") == 4
> number_of_items(in: set) == # number of cards in the set
>
> # the number of red cards in the set
> number_of_items(in: set, filter: { card.color == "red" } )