2.1.0+dune

CHANGES:

• Add Mtime.Span.{is_shorter,is_longer} to make duration
  comparisons more obivous. Thanks to Pau Ruiz Safont for
  the suggestion and the patch.
• Regularize naming structure. The mtime.clock.os library
  is deprecated. Use mtime.clock instead.
• Make the library mtime.clock export mtime.