Python bindings

A Store crosses into Python, through nanobind, as the standard container of its kind does, with nanobind’s own casters:

Store	Python
a sequence	list
a set	set
a map with one value per key	dict

Which Stores convert is decided by their representation. The header in urd/python/ named as the representation’s in urd/repr/ switches it on: a binding includes the caster of each representation its Stores resolve to, as it includes the representation itself.

#include <urd/python/sequences/sequence.hpp>      // Store<sequence<T>>: std::vector
#include <urd/python/maps/unordered_flat_map.hpp>  // Store<map<K, V>>: gtl::flat_hash_map

NB_MODULE(example, m)
{
    m.def("names", [] { return urd::Store<urd::sequence<std::string>> { "on", "clear" }; });
    m.def("arity", [](urd::Store<urd::map<std::string, int>> arity) { return arity.size(); });
}

Casters exist for the sequences (sequence, segmented), the sets (dense_set, flat_set, indexed, parallel_hash_set, sparse_set, std_set, unordered_flat_set) and the maps with one value per key (flat_map, index_map, ordered_map, parallel_hash_map, std_map, unordered_flat_map). A multimap has none, since a dict holds one value per key; nor do the wrappers (recycling, scoped, change-logged), queues, pools and partitions. urd::python::castable<S> says whether a Store converts. The headers need nanobind; nothing else in urd does. The test tests/python/casters.py is built where CMake finds nanobind (-Dnanobind_DIR=...).