Nat.digits() read-only accessor and Nat::Nat({ ...digits sequence...}) ctor

[saxbophone]

• [x] Reading the digits of a Nat object is useful for optimising some algorithms (e.g. ilog(2, ...) can find the highest bit set)
• [x] Creating a Nat object from a sequence of digits could be useful. Such a feature will really come into its own if we ever get around to implementing compile-time pack()/unpack() functionality as described in #63

constexpr const codlili::List<StorageType> digits() const { ... }

constexpr Nat(std::initializer_list<StorageType> digits) { ... }
constexpr Nat(std::span<StorageType> digits) { ... }
// requires Iterable<StorageType>.begin(), Iterable<StorageType>.end() to be valid statements
template <typename Iterable>
constexpr Nat(Iterable<StorageType> digits) { ... }