Document and statically assert counters can't overflow

[torben-hansen]

Description of changes:

2^48 is generally an upper bound on number of CTR-DRBG invocations without reseed. Statically assert type is large enough and for other counters document that 2^64 is the assumed strict upper bound.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.87%. Comparing base (85f783d) to head (ffa0f12).
Report is 1 commits behind head on randomness_generation.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           randomness_generation    #2528      +/-   ##
=========================================================
+ Coverage                  78.86%   78.87%   +0.01%     
=========================================================
  Files                        637      637              
  Lines                     110439   110468      +29     
  Branches                   15622    15622              
=========================================================
+ Hits                       87095    87132      +37     
+ Misses                     22591    22582       -9     
- Partials                     753      754       +1     

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