Partial substitute implemenation

Hi David,

I want to push my view of partial substitutes to the repository.
I use the following syntax:

    // to define default behavior
    var testClass = Substitute.For<TestClass>(ThatCallsBase.ByDefault);
    
    // to specify exact method
    var testClass = Substitute.For<TestClass>();
    testClass.CallBaseFor().VirtualMethod();

I introduced ReturnBaseResultCallHandler into RecordReplay route.
One thing is that I didn't want to complicate everything and haven't put ReturnBaseResultCallHandler and ReturnConfiguredResultHandler on same level.
Currently ReturnConfiguredResultHandler has higher priority than ReturnBaseResultCallHandler regarding argument specification. But it's not difficult to implement, need just to make CallSpecification comparable. The question is do we need it?

Thank you.