[midnight adapter] unordered array

[adhusson]

Simpler to think about than linked list code.

For gas there is an extra cost of having to open <= 10 additional slots over the lifetime of the contract (the max slots used by the array).

Then we get those gas numbers:

approx. average number of cold reads

operation                         linked list             unsorted
----------------------------------------------------------------------
accrual, == 0 maturities reached  ~1                     ~1
accrual, >= 1 maturity reached    ~reached               ~10 + reached
insert new maturity               ~future maturities/2   ~1
delete maturity to 0              ~1                     ~1

[adhusson]

We could have uint120 growth, pack growth with durationCount, remove index, and scan the pending maturities every time we remove one (max 22k gas) to get 1 less slot used per maturity.

Done here #932