memory: Make backend interface more low level

[stotko]

The backend interface of memory expects the dynamic memory type to dispatch to the correct platform-specific call. However, this introduces a cyclic dependency to the high-level memory interface. Furthermore, dispatching should not be part of the backend logic as the backends should only provide the implementations of small building blocks which, in turn, are used to build the entire business logic. Redesign the backend API by splitting the malloc/free/memcpy functions into low-level pieces tied to a specific memory type configuration and move the respective dispatching logic up to the high-level shared implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 97.56%. Comparing base (b751532) to head (f00f105).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/stdgpu/impl/memory.cpp	73.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
- Coverage   97.57%   97.56%   -0.02%     
==========================================
  Files          32       32              
  Lines        2556     2585      +29     
==========================================
+ Hits         2494     2522      +28     
- Misses         62       63       +1     

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