[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 think I agree with @endersstocker, I prefer `beIdenticalTo`.

If you feel strongly about it, I wouldn't mind `beSameInstanceAs`. However, I would want it to read like proper English, so it should be `beTheSameInstanceAs`:

**Grammatically incorrect:** "expect X not to be same instance as"
**Grammatically correct:** "expect X not to be *the* same instance as"

But I definitely prefer `beIdenticalTo`. :laughing: