Function min_safe_u64

app/src/ThirdParty/fast_float.h:1476–1478

If a u64 is exactly max_digits_u64() in length, this is the value below which it has definitely overflowed.

// If a u64 is exactly max_digits_u64() in length, this is
// the value below which it has definitely overflowed.
fastfloat_really_inline constexpr uint64_t min_safe_u64(int base) {
  return int_luts<>::min_safe_u64[base - 2];
}

static_assert(std::is_same<equiv_uint_t<double>, uint64_t>::value,
              "equiv_uint should be uint64_t for double");