Get Nat as sequence of bits (and construct from bits)

[saxbophone]

constexpr codlili::List<bool> Nat::bits() const;

constexpr Nat::Nat(const codlili::List<bool>& bits);

template <std::size_t N>
constexpr Nat::Nat(const std::bitset<N>& bits);

We can use std::bitset to take a shortcut when converting to bits (make a bitset for each digit and read bits out of them in sequence).

Related to #83, #114

[saxbophone]

Dropped from v0.5 milestone —if we're going to read bits, we should probably support writing them (via ctor) too, that is too much work for the meagre benefit right now.