Quick

Forward-declare World, which is now internal

One of Quick's greatest design flaws is due to the combination
of four factors:

1. Quick has a predilection for Swift--one of it's goals was to be
   written entirely in Swift if possible.
2. Quick supports tests written in both Swift and Objective-C.
3. XCTest relies upon subclassing for test discovery.
4. Objective-C classes cannot subclass Swift base classes.

As such, Quick is unable to define QuickSpec, a base XCTestCase
subclass, in Swift, and then have both Objective-C and Swift tests
subclass QuickSpec.

Instead, in order to be used from both Swift and Objective-C, QuickSpec
must be implemented in Objective-C. This forces internal Quick code,
such as Quick.World, to be marked as public. People using Quick to
write tests were thus able to call Quick.World directly, even though
that class is meant to be an implementation detail.

In order to get around the need for World to be public, define macros
which can be used to make a forward declaration of World within an
Objective-C file.