Generate st.fixed_dictionaries() with varied iteration orders

[Zac-HD]

Currently we generate st.fixed_dictionaries() with all the required keys in order, and then a subset of the optional keys after. This was of course fine back when dict iteration order was arbitrary anyway, but it's been insertion-order since Python 3.7 and so we should consider ways to give more varied iteration orders. Thanks to Anne for prompting this issue.

Although if the mapping is an OrderedDict, maybe you want the unvarying iteration order? We might want to add vary_order: bool=True kwarg to control this, inelegant though it seems.

No strong opinions on how we should implement this yet, just that we should think carefully about how whatever-we-do affects the distribution of lengths we generate and thus the ratio of required keys to optional.