use custom 'PagerData' protocol for data to avoid unexpected slowness

[gfreezy]

trafficstars

Do not convert the input data to array to invalidate lazy effects.

Usecase:

Photos PHFetchResult is a lazy object. Converting it to an array cost a lot of time if you have a large number of photos.

[gfreezy]

Seems SwiftUIPager.xcodeproj is useless. We should remove it to avoid confusion