src,test: Fix hicpp-braces-around-statements warnings

[stotko]

Although return statements after if clauses do not require braces, it is still useful to have them for readability reasons. Fix these clang-tidy warnings.

[codecov]

Codecov Report

Merging #132 into master will decrease coverage by 0.28%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
- Coverage   97.92%   97.63%   -0.29%     
==========================================
  Files          28       28              
  Lines        1685     1694       +9     
==========================================
+ Hits         1650     1654       +4     
- Misses         35       40       +5     

Impacted Files	Coverage Δ
src/stdgpu/impl/atomic_detail.cuh	98.44% <50.00%> (-1.56%)	⬇️
src/stdgpu/impl/unordered_base_detail.cuh	96.49% <50.00%> (-0.25%)	⬇️
src/stdgpu/impl/deque_detail.cuh	93.56% <75.00%> (-0.44%)	⬇️
src/stdgpu/impl/vector_detail.cuh	93.87% <75.00%> (-0.61%)	⬇️
src/stdgpu/impl/bit_detail.h	100.00% <100.00%> (ø)

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