Add a Concat slice function.

[FGasper]

This is a variadic wrapper around Flatten that allows callers to assemble one slice from many slices without first joining them into a slice-of-slices.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.50%. Comparing base (f3e6605) to head (16d73a1).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #714   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files          18       18           
  Lines        3437     3439    +2     
=======================================
+ Hits         3248     3250    +2     
  Misses        175      175           
  Partials       14       14           

Flag	Coverage Δ
unittests	94.50% <100.00%> (+<0.01%)	⬆️

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