memory: Add construct_at function

[stotko]

Similar to destroy_at which got introduced in C++17, we are now getting a construct_at function in C++20. Add a respective GPU-callable version of construct_at to simplify the implementation of related classes in the memory module and extend the support of the latter.

[codecov]

Codecov Report

Merging #95 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   97.76%   97.83%   +0.06%     
==========================================
  Files          27       27              
  Lines        1611     1613       +2     
==========================================
+ Hits         1575     1578       +3     
+ Misses         36       35       -1

Impacted Files	Coverage Δ
src/stdgpu/impl/memory_detail.h	98.64% <100%> (+0.01%)	⬆️
src/stdgpu/impl/unordered_base_detail.cuh	96.7% <0%> (+0.25%)	⬆️

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