Issue Description:

In some low-level mathematical operations (e.g. cryptographic applications) it is useful to have a bitwise “rotate” operation. It would be nice if Swift would expose this operation directly, especially as it would be straightforward for Swift to use [the LLVM intrinsics| https://reviews.llvm.org/D50924] that enable optimised codegen on platforms that have appropriate instructions for this task.

As a hypothetical enhancement to this project the rotate operation could even be available as a protocol hook on FixedWidthInteger, with a default implementation available in terms of left shift, right shift, and bitwise or. This would likely be unnecessary, and rarely useful, but not entirely useless.