Multiple stubs with anything

In the process of working on #68359660 and #68359928, we found a number of other cases in which multiple stubbing would not be allowed (or crash!) which really should have worked. This introduces the concept of comparing two stubs' arguments for equality, instead of using the same argument matching as is used when choosing a stub to invoke. It also removes some special casing around Arguments::anything while adding support for using Arguments::anything in different argument positions on multiple stubs.