1.2.0+dune

CHANGES:

• Fix fractional renderings of Ptime.Span.pp with leading zeros. For
  example 1.036s would render as 1.36s. This is a rendering bug in a
  function for human display, not a bug in the computations or
  conversion functions of Ptime.
• Add Ptime.weekday type for naming the result of the Ptime.weekday
  function.
• Regularize naming structure. The ptime.clock.os library is deprecated.
  Use ptime.clock instead.
• Make the library ptime.clock export ptime.