memory: Cleanup global variables and simplify allocate/deallocate logic

[stotko]

The safe memory system currently relies on global variables for atomics and mutexes as well as for the internal allocation_manager class. However, global variables typically indicate bad design and should be avoided in general. Get rid of all of them by moving the instances of allocation_manager to the internal dispatch_allocation_manager function, which now also acts as the singleton function, and by significantly simplifying the internal allocate and deallocate logic.

[codecov]

Codecov Report

Merging #104 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
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  Coverage   97.83%   97.83%           
=======================================
  Files          27       27           
  Lines        1613     1613           
=======================================
  Hits         1578     1578           
  Misses         35       35

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