Adds matchers for Swift 5 Result type

[xzeror]

Swift 5 adds common Result type. It's useful to have matchers out of the box in Nimble.
This PR adds matchers beSuccess and beFailure. This is purely additive changes with full test coverage. Also added some code samples into Readme.

• [✅ ] Does this have tests?
• [✅ ] Does this have documentation?
• [ ❌] Does this break the public API (Requires major version bump)?
• [✅ ] Is this a new feature (Requires minor version bump)?

[dtrenz]

For what it's worth, I'm using these as my local custom Result type matchers, in case someone wants to steal them:

func beFailure<Success, Failure>() -> Predicate<Result<Success, Failure>> {
    return Predicate.simple("be failure") { actualExpression in
        guard let actual = try actualExpression.evaluate(), case .failure = actual else {
            return .doesNotMatch
        }
        return .matches
    }
}

func beSuccess<Success, Failure>() -> Predicate<Result<Success, Failure>> {
    return Predicate.simple("be success") { actualExpression in
        guard let actual = try actualExpression.evaluate(), case .success = actual else {
            return .doesNotMatch
        }
        return .matches
    }
}

[ikesyo]

Sorry for the late, I'm rebooting this now for Nimble v9.

[ikesyo]

I'll tackle the feedbacks myself.