relates to:

• SR-2016 stride(from:0 as UInt8, though: 255, by: 2) should not trap.
• SR-13883 StrideToIterator and StrideThroughIterator are broken

Issue Description:

Current implementation of Stride(To|Through)Iterable relies on passing beyond the upper bound parameter _end in order to detect the completion of the iteration.

Given:

enum E : Int, Strideable {
   case one = 1, two, three, four
   func distance(to other: Self) -> Int {
      return other.rawValue - self.rawValue
   }
   func advanced(by n: Int) -> Self {
      return Self(rawValue: self.rawValue + n)!
   }
}

it is impossible to iterate to or through the last value, as it'd require a non-existant five (or a catch-all-unknown value) to exist in order to detect the completion of the iteration.

It is for that reason that this calls crashes when .three is attempted to be advanced by 2:

print(Array(stride(from: E.one, to: E.four, by: 2)))

The resolution of SR-2016 SR-13883 solves a very similar issue when trying to deal with numeric type's frontiers by relying on FixedWidthInteger's min and max. This ticket is to find a way to extend this further so other types can stridden without requiring to represent otherwise-impossible values.