Fix stream ops for RNG instances

[Flamefire]

Users might want to test if reading or writing the RNG state worked
The only way is checking the stream state.

So add assertions for that and fix the offending ones

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.79%. Comparing base (f1983c8) to head (74d6d20).

Files with missing lines	Patch %	Lines
include/boost/random/mixmax.hpp	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #156      +/-   ##
===========================================
- Coverage    95.79%   95.79%   -0.01%     
===========================================
  Files          117      117              
  Lines         7660     7652       -8     
===========================================
- Hits          7338     7330       -8     
  Misses         322      322              

Files with missing lines	Coverage Δ
include/boost/random/lagged_fibonacci.hpp	98.96% <100.00%> (ø)
include/boost/random/mersenne_twister.hpp	99.43% <100.00%> (-0.01%)	⬇️
include/boost/random/splitmix64.hpp	100.00% <100.00%> (ø)
test/test_generator.ipp	100.00% <100.00%> (ø)
include/boost/random/mixmax.hpp	99.04% <90.00%> (-0.02%)	⬇️

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[mborland]

This looks good, but currently has a merge conflict.

[Flamefire]

Rebased. CI passes again