[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.
This message should also mimic the `Equal` matcher:

```swift
expected subject not to be identical to 'Sandor Clegane' (0x0001)
```

And the forced unwrapping comments above also apply here as well.