Support HRTB and various fixes

Added

• Allow lifetime for<'a, ...> bounds in non-bounded generic parameters.

Changed

• Use the Copy implementation for Clone and the Ord implementation for
  PartialOrd when custom bounds are present.

Fixed

• Support skipping only some variants with ZeroizeOnDrop.
• Only generate single call to zeroize() with ZeroizeOnDrop on multiple
  fields or variants.