[Nimble] Add matcher `BeSameInstanceAs()`

Adds matcher `BeSameInstanceAs()`, which compares two object instances to see if they are the identical instance.

If either or both instances are nil, `BeSameInstanceAs()` returns false. Ideally, `BeSameInstanceAs(nil)` should raise an exception, but as of right now, there doesn't seem to be a way (that I can figure out) of testing that, so for right now, it simply returns false.
Same as above, you can just `return "expected subject not to be nil"` here.