Add widening policy for add and mul

[mborland]

Closes: #20

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (784c311) to head (c09ec59).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop     #34   +/-   ##
=======================================
  Coverage     98.9%   98.9%           
=======================================
  Files           81      81           
  Lines         4371    4371           
  Branches      1350    1350           
=======================================
  Hits          4322    4322           
  Misses          22      22           
  Partials        27      27           

Files with missing lines	Coverage Δ
...ost/safe_numbers/detail/unsigned_integer_basis.hpp	94.6% <ø> (ø)

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