Update SortedMerge to use a PriorityQueue

[danielearwicker]

Fixes #654

[viceroypenguin]

comments are mostly style nits - if you could fix other single line blocks to not use {} as well, that would be good. also, there's a build error, if you could look at that please.

[danielearwicker]

Looks like Test.Async.EndsWithTest.EndsWithWithStrings has some intermittent timing issue?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.46%. Comparing base (ef180fd) to head (4553ec0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #657      +/-   ##
==========================================
- Coverage   95.60%   95.46%   -0.14%     
==========================================
  Files         247      247              
  Lines        8806     8867      +61     
  Branches     1602     1620      +18     
==========================================
+ Hits         8419     8465      +46     
- Misses        193      210      +17     
+ Partials      194      192       -2     

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[viceroypenguin]

Thanks!