Map matcher that inverts the submatcher.

[younata]

We have the normal map matcher. But what about when you want to invert the submatcher, e.g. you want to check if the map'd value is not nil?

Be sure to also update the failure message, to at least insert a "not" in front of the message.

While we're at it, we should probably create an alias of map that makes the matcher argument a named argument. e.g. map(_:to:)