Fix for Swift 5 compiler

• The Swift 5 compiler no longer imports structs with large alignments.
  This defeats the approach I previously took to pad structs.
• In order to get padded structs to mitigate false sharing effects, the package
  now inserts explicit padding rather than explicitly requesting a large alignment.
  This is not the ideal solution, but it's good enough for most cases.
• The padded types are renamed from 4.4.0, with abject apologies to anyone
  who might be affected. I should have tried building with Swift 5 a tad earlier.