cartesian_product size() can overflow

[tcbrindle]

As noted when @isaacy2012 was adding cartesian_power, the current implementation of size() for cartesian_product[_map] uses built-in multiplication in a fold expression, and could easily overflow:

flux/include/flux/op/cartesian_base.hpp

Lines 219 to 227 in 85b016e

	template <typename Self>
	static constexpr auto size(Self& self) -> distance_t
	requires (CartesianKind == cartesian_kind::product
	&& (sized_sequence<Bases> && ...))
	{
	return std::apply([](auto&... base) {
	return (flux::size(base) * ...);
	}, self.bases_);
	}

It should use num::checked_mul() instead to guard against UB.