Fix infinite loop in swizzler

[mmaatttt]

I discovered that one (but perhaps not the only?) way to cause the swizzling to infinite loop is with inheritance. The test case in this PR shows it, but what could happen is:

1. swizzle Delegate1 scrollViewDidEndDragging
    a. add placeholder impl
    b. add actual impl
    c. swap placeholder and actual
2. swizzle Delegate2 scrollViewDidEndDragging
    a. add actual impl
    b. swap with "original", which is actually itself (from 1.b) 

Checking that the actual implementations that we intend to swap are different is the key here.

In my investigation and testing, I also notice we were trying to swizzle the same class multiple times. The guard checks in the Swizzler prevent anything bad from happening, but it's still unnecessary, so I added a Set to track what's been swizzled to prevent repeat attempts that could potentially cause something to break.

And for the case where a scroll view has a nil delegate, we can just assign it AppcuesScrollViewDelegate.shared with the implementation we need instead of also swizzling the AppcuesScrollViewDelegate.shared since we own its implementation. This just saves us from more swizzling in this one case, making things a bit safer overall.

This should fix #582

[iujames]

great find and improvements!