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 78.70%. Comparing base (e4c4aa1) to head (15dae6e).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           randomness_generation    #2528      +/-   ##
=========================================================
- Coverage                  78.70%   78.70%   -0.01%     
=========================================================
  Files                        661      661              
  Lines                     112927   112927              
  Branches                   15902    15895       -7     
=========================================================
- Hits                       88877    88876       -1     
+ Misses                     23278    23277       -1     
- Partials                     772      774       +2     

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[torben-hansen]

Use instead #2658