Demo app fails to build in Xcode 12

[NickEntin]

When building the demo app with the Xcode 12 GM, the compiler has trouble resolving the type of expressions:

Example/Stagehand/CollectionKeyframesViewController.swift:93:27: The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions
Example/Stagehand/CollectionKeyframesViewController.swift:99:27: The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions
Example/Unit Tests/CGAffineTransformInterpolationSnapshotTests.swift:59:9: Type of expression is ambiguous without more context

[NickEntin]

Since Collection Keyframes is an incomplete feature and no progress has been made recently, I think it's reasonable to remove that view controller entirely for now (or maybe remove the entire feature). The other issue should be easy to address.

• Remove collection keyframes demo screen
• Provide the compiler with explicit types in affine transform snapshot tests