Some new methods. A precondition fix. Cleanup n' stuff.
- #88
quotientAndRemainderReportingOverflow(dividingBy:)
- #87
Int256.min.quotientAndRemainder(dividingBy: -1) should trap
- #86 ANKFixedWidthInteger.BitPattern
- #84 Lazy division overflow detection
- #82 Arithmetic protocol extensions
- #81 Truncating multiplication operators
