Expose Mock.Setups, part 6: Replace OriginalSetup with simpler OriginalExpression

[stakx]

(This is a follow-up to #995.)

There are several problems with the current ISetup ISetup.OriginalSetup { get; }:

1. The IFluentSetups that can be returned via this property aren't really working setups; they're inert data objects used mostly for introspection. This means we have several things in the code base that call themselves "setups", yet are basically apples and oranges.

2. Setup expressions are always lambdas. This may be too restrictive if at a later time we e.g. want to link stubbed property accessor setups to a specific mock.SetupAllProperties() call.

3. IFluentSetup may simply be quite difficult to understand; also, it might not be obvious that there's more functionality available when explicitly casting OriginalSetup to IFluentSetup.

For those reasons, let's replace OriginalSetup with a much simpler solution: a Expression OriginalExpression { get; } property, which should be sufficient in most scenarios and is quite easy to grasp.