[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.
> I removed the tests around the failure messages.

Hmm, you may have added them back when you resolved some merge conflicts or something--they're in the diff, and as a result the tests fail on your branch. Why not mark the tests as pending, so we remember to test them in the future?

```swift
describe("failureMessage") {
    pending("says it expected the instances to be the same") {
        let message = matcher.failureMessage("Sandor Clegane")
        expect(message).to.equal("expected 'Sandor Clegane' (0x00...), got 'Sandor Clegane' (0x00...)")
    }
}

describe("negativeFailureMessage") {
    pending("says it expected actual to not be the same instance as expected") {
        let message = matcher.negativeFailureMessage("Kingsguard")
        expect(message).to.equal("expected subject not to be the same instance as 'Sandor Clegane' (0x00...)")
    }
}
```