BUG: Fix bug identified in uint8 and uint16 implementations of Lemire

[bashtage]

Avoid upcasting when negating small unsigned values

[codecov]

Codecov Report

Merging #187 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files          37       37           
  Lines       10200    10200           
  Branches      154      154           
=======================================
  Hits         9694     9694           
  Misses        482      482           
  Partials       24       24

Impacted Files	Coverage Δ
randomgen/tests/test_final_release_changes.py	100% <100%> (ø)	⬆️

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