Touch the oldest completed slice when marking a burrow for liquidation

[gkaracha]

Closes #155.

I am opening this as a draft PR already so that it's easier to evaluate the gas cost differences.

By looking at this I realized that we have quite some code duplication around touching liquidation slices. touch_liquidation_slice is the function processing a single slice, but then we have

1. touch_oldest that touches the oldest Constants.number_of_slices_to_process completed slices,
2. touch_liquidation_slices_rec that touches a list of completed slices passed as an argument, and now
3. entrypoint_mark_for_liquidation also finds the oldest completed slice and then touches it.

I am thinking that perhaps this is a good time to restructure the code a little to reduce duplication, as part of this PR. If we implement find_oldest_slices : Ligo.nat -> liquidation_auctions -> leaf_ptr list, then (1) and (3) can be simplified quite some:

• touch_oldest can be rewritten (inlined, really) as touch_liquidation_slices_rec (find_oldest_slices Constants.number_of_slices_to_process auctions), and
• entrypoint_mark_for_liquidation can now also simply call touch_liquidation_slices_rec (find_oldest_slices 1 auctions).

I'll have a go and see if gas costs allow for these changes.

[gkaracha]

Sidenote: experiment of the find_oldest_slices/touch_slices separation here. By now I wonder if any of these changes is really possible, given gas costs.

[dorranh]

Gas costs	335029d	4849a4b	Diff
touch	537452	537403	-49
buy_kit	69339	69341	2

Entrypoint sizes	335029d	4849a4b	Diff
mark_for_liquidation	17093	33792	16699
touch	56590	56586	-4

Test coverage	335029d	4849a4b	Diff
checker.ml	90.18	89.82	-0.36000000000001364
TOTAL	93.93	93.85	-0.0800000000000125

[dorranh]

Gas costs	335029d	4849a4b	Diff
touch	537452	537403	-49
buy_kit	69339	69341	2

Entrypoint sizes	335029d	4849a4b	Diff
mark_for_liquidation	17093	33792	16699
touch	56590	56586	-4

Test coverage	335029d	4849a4b	Diff
checker.ml	90.18	89.82	-0.36000000000001364
TOTAL	93.93	93.85	-0.0800000000000125

[dorranh]

Gas costs	335029d	4849a4b	Diff
touch	537452	537403	-49
buy_kit	69339	69341	2

Entrypoint sizes	335029d	4849a4b	Diff
mark_for_liquidation	17093	33792	16699
touch	56590	56586	-4

Test coverage	335029d	4849a4b	Diff
checker.ml	90.18	89.82	-0.36000000000001364
TOTAL	93.93	93.85	-0.0800000000000125

[dorranh]

Gas costs	335029d	4849a4b	Diff
touch	537452	537403	-49
buy_kit	69339	69341	2

Entrypoint sizes	335029d	4849a4b	Diff
mark_for_liquidation	17093	33792	16699
touch	56590	56586	-4

Test coverage	335029d	4849a4b	Diff
checker.ml	90.18	89.82	-0.36000000000001364
TOTAL	93.93	93.85	-0.0800000000000125