The prime-testing techniques we will use (Miller-Rabin, Strong Lucas)
all make heavy usage of modular arithmetic. Therefore, we lay those
foundations here, adding utilities to perform the basic arithmetic
operations robustly.
Since these are internal-only helper functions, we don't bother checking
the preconditions, although we state them clearly in the contract
comment for each utility. After C++26, we could add contracts for
these.
Helps #509.
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