Fix promotion system with inclusion of decimal32_fast

[mborland]

Closes: #588

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (d901dd4) to head (09b93df).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #589   +/-   ##
=======================================
  Coverage     98.9%   98.9%           
=======================================
  Files          200     200           
  Lines        12912   12912           
  Branches      1487    1487           
=======================================
  Hits         12766   12766           
  Misses         146     146           

Files	Coverage Δ
test/test_promotion.cpp	100.0% <ø> (ø)

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