Prevent deadlock when purging idle worker during task submission

[alitto]

• Fix for https://github.com/alitto/pond/issues/33
• Upgrade to go 1.19
• Extracted counter updates from main worker function to make it simpler and more generic
• Moved worker function to a separate file
• Added Makefile with test targets

[codecov-commenter]

Codecov Report

Merging #34 (7edfde9) into master (d01340a) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #34   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         4    +1     
  Lines          376       394   +18     
=========================================
+ Hits           376       394   +18     

Impacted Files	Coverage Δ
pond.go	100.00% <100.00%> (ø)
worker.go	100.00% <100.00%> (ø)

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