Simplify numeric_limits and fix ODR violations

[mborland]

Closes: #1203

[codecov]

Codecov Report

❌ Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.1%. Comparing base (c6dd35a) to head (c591ec5).
⚠️ Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
test/limits_link_1.cpp	0.0%	10 Missing ⚠️
test/limits_link_2.cpp	0.0%	10 Missing ⚠️
test/limits_link_3.cpp	0.0%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1220     +/-   ##
=========================================
- Coverage     99.2%   99.1%   -0.1%     
=========================================
  Files          257     260      +3     
  Lines        17134   17158     +24     
  Branches      1935    1935             
=========================================
  Hits         16994   16994             
- Misses         140     164     +24     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.2% <ø> (ø)
include/boost/decimal/decimal32_t.hpp	96.6% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	95.2% <ø> (ø)
include/boost/decimal/decimal_fast128_t.hpp	96.0% <ø> (ø)
include/boost/decimal/decimal_fast32_t.hpp	100.0% <ø> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <ø> (ø)
test/limits_link_3.cpp	0.0% <0.0%> (ø)
test/limits_link_1.cpp	0.0% <0.0%> (ø)
test/limits_link_2.cpp	0.0% <0.0%> (ø)

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