Safer atomics usage

[guregu]

I'm not an expert on atomics, but my understanding is that if you're doing atomic writes on an object you need to use atomics for reads too. I changed NewVariable to use atomic.AddUint64's return value instead of accessing the counter variable directly.

Feel free to ignore this if it's wrong. I tried to look up whether accessing global value types non-atomically after atomic writes was safe or not but I couldn't find much, but my gut instinct is that it's probably best avoided 🤔

[codecov-commenter]

Codecov Report

Merging #208 (1382d65) into main (f89c74e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #208   +/-   ##
=======================================
  Coverage   96.68%   96.68%           
=======================================
  Files          20       20           
  Lines        6244     6244           
=======================================
  Hits         6037     6037           
  Misses        170      170           
  Partials       37       37           

Impacted Files	Coverage Δ
engine/variable.go	95.23% <100.00%> (ø)

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[ichiban]

This is exactly what I intended and failed. Looks great!