resolve SendItems

[plastikfan]

SendItems has been modified to accept a variadic collection of Items rather than T. This means that we now have to invoke in the foloowing way:

	rx.SendItems(context.Background(), ch, rx.CloseChannel,
		rx.Of(1),
		rx.Of(2),
		rx.Of(3),
	)

this is in contrast to to rxgo:

	rx.SendItems(context.Background(), ch, rx.CloseChannel,
		1, 2, 3,
	)

This issue is complicated by the fact we need to be able to represent a stream of values that may include an error. Using rxgo, we can effectively do this:

	rx.SendItems(context.Background(), ch, rx.CloseChannel,
		1, someErr, 3,
	)

but our modified version will not support this because error, can't be sent down a channel of T.

[plastikfan]

Actually, the original description for this item is incorrect. We are not forced to use Of to send items, in fact doing so this way degrades the DX. We will and should not enforce this.