Improvements to decimal64_fast

[mborland]

Normalize the values in the constructor and remove all the calls to normalize in the basis functions

Closes #639

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.6%. Comparing base (b4b08c2) to head (4e0b209).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #644     +/-   ##
=========================================
- Coverage     98.7%   98.6%   -0.0%     
=========================================
  Files          219     219             
  Lines        15717   15636     -81     
  Branches      1720    1719      -1     
=========================================
- Hits         15497   15414     -83     
- Misses         220     222      +2     

Files	Coverage Δ
include/boost/decimal/decimal64_fast.hpp	98.4% <100.0%> (-0.7%)	⬇️
include/boost/decimal/detail/add_impl.hpp	100.0% <100.0%> (ø)
test/test_decimal64_fast_basis.cpp	100.0% <100.0%> (ø)

... and 13 files with indirect coverage changes

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