memory: Add (de)allocatate_filled functions to allocator_traits

[stotko]

Although the memory module already supports custom allocators making it quite generic, the more advanced API related to automatically filled arrays still uses the default execution policies. Extend allocator_traits by allocate_filled and deallocate_filled functions that additionally take an execution policy as well as an initialization value as their arguments. This consolidates all required allocation functions in the traits and follows the recommended usage from the C++ standard.

Issue: #351

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (bc3adae) 97.23% compared to head (914894e) 97.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   97.23%   97.24%   +0.01%     
==========================================
  Files          31       31              
  Lines        2421     2430       +9     
==========================================
+ Hits         2354     2363       +9     
  Misses         67       67              

Impacted Files	Coverage Δ
src/stdgpu/impl/memory_detail.h	99.19% <100.00%> (+0.03%)	⬆️

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