Vary iteration order of fixed_dictionaries output

Since dict iteration order has been observable since Python 3.7,
fixed_dictionaries now shuffles its key/value pairs (after choosing which
optional keys to include, so only order varies) to help find bugs in code
sensitive to key order. Examples still shrink towards the original order.

The shuffle is shared with PermutationStrategy via a new
fisher_yates_shuffle(data, ls) helper in conjecture utils.

https://claude.ai/code/session_01MUo34UvHAsURLeBXDmsx29