Add facility for hiding values from the compiler

[dabrahams]

/// Returns `x`, preventing its value from being statically known by the optimizer.
@inline(__always)
fileprivate func hideValue<T>(_ x: T) -> T {
  
  @_optimize(none) 
  func assumePointeeIsWritten<T>(_ x: UnsafeMutablePointer<T>) {}
  
  var copy = x
  withUnsafeMutablePointer(to: &copy) { assumePointeeIsWritten($0) }
  return copy
}