Refactor: Have a common base for Kyber/Dilithium structures

[reneme]

... very much work in progress.

[coveralls]

coverage: 92.002% (-0.02%) from 92.021%
when pulling 64f9db8 on Rohde-Schwarz:refactor/crystals_april
into b1a6d25 on randombit:master.

[reneme]

clang's UBsan is complaining about a signed integer overflow in Kyber's montgomery reduction. Did we drop a reduce() somewhere?

/home/runner/work/botan/botan/build/build/include/internal/botan/internal/kyber_constants.h:50:42: runtime error: signed integer overflow: 3611157 * -3327 cannot be represented in type 'int'
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/runner/work/botan/botan/build/build/include/internal/botan/internal/kyber_constants.h:50:42 in 
  Ran for 354 seconds

[reneme]

FYI: I won't continue working on this before end of May. ⏳