Consider preserving order of `sexpr`ed maps

[lread]

Problem/Opportunity

We just merged #419 which perserves sexpr map element order for small maps (just like Clojure happens to do).

But maybe we could always (or optionally) preserve sexpr maps regardless of map size.

Option 0: Don't do anything

We are good with #419 behaviour.

Pros:

• no new code, no new bugs!

Cons:

• sexpr map order is preserved only for small maps

Option 1: Always preserve map order

Pros:

• No additional knobs or dials to turn

Cons:

• This is a perf impact for larger maps with no way to opt out

Option 2: Add an option flag

Add an option to preserve sexpr map element order.
Maybe :preserve-map-elem-order?

Pros:

• There is a performance impact for larger maps, this would let the user choose.

Cons:

• Granularity of choice is not tweakable at map level.

Option 3: Add an option fn

Add an option to specify a map fn
Maybe :sexpr-map-fn.

Pros:

• User can tweak choice at map level

Cons:

• More complicated to use than a flag.

Additional Notes

• default behaviour would remain unchanged
• option would be available at node sexpr level fns and zipper creation fns.
• this does not cover preserving order of sets, we are just talking about maps here.

Next Steps

Mull over choices, flesh out details.
If we were to proceed, I think Option 3 might be interesting to explore.

[borkdude]

I'd vote for 3 since then you can even provide an ordered map implementation which is better for round-tripping and safer to use than an array map (since calling assoc on the latter may turn it back into a hash-map, etc).
With 2: I kind of dislike keywords with question marks for boolean options, but this is just my taste.