1.6

CHANGES:

• Use the standard inf and sup for the bounds of the interval.
• Prefer ..._up and ..._dw to mark the rounding of functions and
  RoundUp and RoundDown for the modules. Crlibm was updated
  accordingly.
• Rename Interval as Interval_base as it conflicted with the
  Interval module in compiler libs (provoking a collision when this
  library was used in the REPL).
• Add functions mag (magnitude) and mig (mignitude), mid
  (midpoint), singleton (creation of intervals containing a single
  value), is_singleton, diam (diameter), belong (x ∈ I), floor,
  ceil and hypot.
• Renamed Interval_*.t as Interval_*.interval as to not import the
  short type t into the namespace (but still have access to the
  record fields).
• Add functions RoundDown.dist and RoundUp.dist (downward/upward
  rounded distance).
• Use Dune 2.0.