[OUTDATED] Add iterable concept

[tcbrindle]

This PR adds a new iterable concept and updates various algorithms and adaptors to use it.

An iterable is, as the name suggests, something we can iterate over -- specifically, by using internal iteration. The required interface is:

struct sequence_traits<T> {
    auto element_type(T& iterable) -> E;

    template <typename T, typename Pred>
        requires std::predicate<Pred&, E>
    auto iterate(T& iterable, Pred&& pred) -> bool;
};

This is basically the existing for_each_while abstracted out into its own concept, returning a bool to indicate whether iteration was completed successfully rather than a cursor.

Once iterate() has returned, it is unspecified whether a second call to iterate() will start again from the beginning, carry on where it left off, do nothing, or do something else. Iterables are strictly weaker than sequences: that is, every sequence is an iterable, but not vice versa. Nonetheless, a large number of algorithms work on iterables (just about anything that can be written as a short-circuiting fold), and most of the existing sequence adaptors can work on iterables as well.

Since iterables only use internal iteration, there is no external state (i.e. a cursor) which could be invalidated. Not only does this make the iterable interface easier to implement for some types, but most importantly it means that we can provide a safe default implementation of the iterable protocol for all C++20 ranges.

This dramatically improves our interoperability with ranges, while the use of internal iteration means that converting ranges pipelines to the equivalent Flux code may yield some nice performance benefits.