ValueIDNum is supposed to be a value type that boils down to a uint64_t,
that has some bitfields for convenience. If we use the default operator=,
we end up with each bit field being individually assigned, which is
un-necessarily slow.
Implement the assignment operator by just copying the uint64_t value of
the object. This is quicker, and matches how the comparison operators
work already. Doing so is 0.1% faster on the compile-time-tracker.
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