Use an explicit type annotation here

[michaelklishin]

References #186.

One hypothesis in #186 was that compiler's type inference fails to determine that the value should be a usize. Since AtomicUsize#fetch_add returns a usize, I highly doubt it is a compiler issue. Instead, the cloning/concurrency factors are more important.

That said, this was trivial to put together, so why not use this PR to verify the hypothesis.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.70% 🎉

Comparison is base (f335c59) 86.80% compared to head (b904487) 87.51%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   86.80%   87.51%   +0.70%     
==========================================
  Files          68       68              
  Lines        5503     5503              
==========================================
+ Hits         4777     4816      +39     
+ Misses        726      687      -39     

Files Changed	Coverage Δ
src/producer.rs	80.04% <100.00%> (ø)

... and 9 files with indirect coverage changes

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[michaelklishin]

This was to demonstrate that this change isn't necessary and prove a certain hypothesis wrong => closing.