Change values of fast nans and infs to reduce comparisons

[mborland]

Closes: #814

This micro-optimization was already in decimal128_fast

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (1feeee0) to head (64797e7).
Report is 15 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #820     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          230     230             
  Lines        17067   17060      -7     
  Branches      1807    1807             
=========================================
- Hits         16876   16869      -7     
  Misses         191     191             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal32_fast.hpp	96.5% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal64_fast.hpp	98.1% <100.0%> (-<0.1%)	⬇️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1feeee0...64797e7. Read the comment docs.