Single statement operator usage

The current implementation of matcher operators cannot be used in single statement it-closures because they return Bool.

Those operators look like comparison operators, but they aren't and they don't override the comparison operators because Expectation is neither Equatable nor Comparable. They also don't need to return anything and what they returned up till now was also ignored. So this change makes the operators return Void and with this they work in single statement it-closures.

Note that this pull request is a bit messed up as I created my earlier pull request concerning beIdenticalTo with objects on the master branch.