examples,src,test: Fix clang-tidy-8 warnings

[stotko]

Recent versions of clang-tidy contain a larger set of diagnostics. Fix these newly appearing warnings to further improve the code quality.

[codecov]

Codecov Report

Merging #140 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #140   +/-   ##
=======================================
  Coverage   97.64%   97.64%           
=======================================
  Files          28       28           
  Lines        1698     1698           
=======================================
  Hits         1658     1658           
  Misses         40       40           

Impacted Files	Coverage Δ
src/stdgpu/impl/atomic_detail.cuh	98.44% <ø> (ø)
src/stdgpu/impl/unordered_base.cuh	100.00% <ø> (ø)
src/stdgpu/ranges.h	100.00% <ø> (ø)
src/stdgpu/impl/bit_detail.h	100.00% <100.00%> (ø)
src/stdgpu/impl/cmath_detail.h	100.00% <100.00%> (ø)
src/stdgpu/impl/ranges_detail.h	100.00% <100.00%> (ø)
src/stdgpu/impl/unordered_base_detail.cuh	96.50% <100.00%> (ø)

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