Explore rational/fraction design space

[johnmcfarlane]

Attempt to reconcile P1438 with P1050.
Populate this spreadsheed.

Attributes:

• struct (no invariants) vs class (invariants)
• class template vs class

Modes:

• Accurate result, mustn't fail: fraction<int>
• Accurate result but may fail: fraction<safe_integer<>>
• Approximate result, fails less: rational
  fraction with reduction or (failing that) downscaling
• Accurate result and won't fail: rational<unbounded<>>
  • eager
  • lazy (doesn't always store reduced values)