We now use a generic Rand structure which abstracts the core functions
from the backing engine.
The old Mersenne Twister engine is removed and replaced instead with
three alternatives:
- Pcg32
- Xoroshiro128+
- Isaac64
These should provide sufficient coverage for most purposes, including a
CSPRNG using Isaac64. Consumers of the library that do not care about
the actual engine implementation should use DefaultPrng and DefaultCsprng.
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