[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.
Since the `"expected subject not to be nil"` strings are constant, it's possible to test them--and they most certainly should be tested.