Fix left and right arrows in mhchem

[dpvc]

This PR fixes a problem with the mhchem extension where \leftarrow and \rightarrow inside \ce{} are reversed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.72%. Comparing base (afaf1ea) to head (e879a48).
⚠️ Report is 43 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1358     +/-   ##
==========================================
  Coverage    86.72%   86.72%             
==========================================
  Files          337      337             
  Lines        84145    84145             
  Branches      3140     4769   +1629     
==========================================
  Hits         72971    72971             
+ Misses       11174    11151     -23     
- Partials         0       23     +23     

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[zorkow]

lgtm.