src: Use shorter inline variables for detail type traits

[stotko]

While the use of type traits from the C++ standard library have been simplified in #333, our own implementation-specific traits are still referenced by the classic *::value notation. Port these traits to the newer *_v versions as well for consistency and better readability.

Partially addresses #314

[codecov]

Codecov Report

Base: 97.14% // Head: 97.14% // No change to project coverage 👍

Coverage data is based on head (0ec3ad2) compared to base (aecccfd).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #335   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files          32       32           
  Lines        2346     2346           
=======================================
  Hits         2279     2279           
  Misses         67       67           

Impacted Files	Coverage Δ
src/stdgpu/impl/unordered_base.cuh	100.00% <ø> (ø)
src/stdgpu/impl/unordered_base_detail.cuh	96.00% <ø> (ø)
src/stdgpu/impl/unordered_map_detail.cuh	100.00% <ø> (ø)
src/stdgpu/impl/unordered_set_detail.cuh	100.00% <ø> (ø)
src/stdgpu/impl/vector_detail.cuh	94.00% <ø> (ø)
src/stdgpu/unordered_map.cuh	100.00% <ø> (ø)
src/stdgpu/unordered_set.cuh	100.00% <ø> (ø)
src/stdgpu/vector.cuh	100.00% <ø> (ø)
src/stdgpu/impl/memory_detail.h	99.03% <100.00%> (ø)

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