Allow LoopVectorization 0.7 and 0.8.

[chriselrod]

This will solve:
https://discourse.julialang.org/t/trying-to-speed-up-laplace-equation-solver/43862/14?u=elrod

If it is possible for the arrays to be empty, then supporting LoopVectorization 0.8 will require adding check_empty=true as an argument to @avx.

Tests passed locally.

[coveralls]

Coverage remained the same at 96.36% when pulling 2d378e1 on chriselrod:master into 689d8e2 on PetrKryslUCSD:master.

[PetrKryslUCSD]

The package was upgraded to 0.8. Thank you very much.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.35%. Comparing base (689d8e2) to head (2d378e1).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   96.35%   96.35%           
=======================================
  Files          32       32           
  Lines        3956     3956           
=======================================
  Hits         3812     3812           
  Misses        144      144           

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