[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 like [the message](https://github.com/modocache/Quick/blob/d4eddb74bb4abac28c358e4612a39828fc9b8158/Nimble/Nimble/Matchers/Numbers/NumberComparisonMatcher.swift#L12) used in the `BeGreaterThan`, `BeLessThan`, etc., matchers: `"expected subject not to be nil"`.