CompatHelper: bump compat for "DataStructures" to "0.18"

[github-actions]

This pull request changes the compat entry for the DataStructures package from 0.17 to 0.17, 0.18.

This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[coveralls]

Coverage remained the same at 88.467% when pulling 290ac27 on compathelper/new_version/2020-08-18-00-43-37-494-3734050997 into 60e1487 on master.

[codecov]

Codecov Report

Merging #55 (290ac27) into master (60e1487) will not change coverage.
The diff coverage is n/a.

❗ Current head 290ac27 differs from pull request most recent head 6025285. Consider uploading reports for the commit 6025285 to get more accurate results

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   88.40%   88.40%           
=======================================
  Files          51       51           
  Lines        2587     2587           
=======================================
  Hits         2287     2287           
  Misses        300      300           

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