Iterating hash tables gives an undefined order. Often we want to have
a stable order, for example when printing the content of a hash or
when converting it to a "a{sv}" variant.
How to achieve that best? I think we should only iterate the hash once,
and not require additional lookups. nm_utils_named_values_from_strdict()
achieves that by returning the key and the value together. Also, often
we only need the list for a short time, so we can avoid heap allocating
the list, if it is short enough. This works by allowing the caller to
provide a pre-allocated buffer (usually on the stack) and only as fallback
allocate a new list.
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