All members of equivalence class should return the same hash value

[mborland]

Closes: #1120

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (4d0c7e2) to head (b0a022f).
⚠️ Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1123     +/-   ##
=========================================
+ Coverage     99.4%   99.4%   +0.1%     
=========================================
  Files          246     246             
  Lines        16778   16793     +15     
  Branches      1812    1814      +2     
=========================================
+ Hits         16671   16686     +15     
  Misses         107     107             

Files with missing lines	Coverage Δ
include/boost/decimal/hash.hpp	100.0% <100.0%> (ø)
test/test_hash.cpp	100.0% <100.0%> (ø)

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