Signed saturating arithmetic and docs

[mborland]

Closes: #146
Closes: #148

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[codecov]

Codecov Report

Attention: Patch coverage is 99.59514% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.09%. Comparing base (ca8c3a9) to head (0d6e30e).
Report is 53 commits behind head on master.

Files with missing lines	Patch %	Lines
test/test_saturating_arith.cpp	99.46%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   94.52%   95.09%   +0.57%     
==========================================
  Files          28       28              
  Lines        3909     4141     +232     
  Branches     1020     1116      +96     
==========================================
+ Hits         3695     3938     +243     
+ Misses         74       62      -12     
- Partials      140      141       +1     

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