Make tictac more efficient by making level1 a map

[martinsumner]

Pre-change (1M keys, tree size large):

Generating Keys took 2513 milliseconds
Memory footprint [{total,356732576},{processes,334051328},{processes_used,334044488},{system,22681248},{atom,540873},{atom_used,524383},{binary,1015120},{code,9692859},{ets,721496}]
Generating new tree took 1 milliseconds
Loading tree took 27967 milliseconds
Memory footprint [{total,36733040},{processes,8875472},{processes_used,8875048},{system,27857568},{atom,540873},{atom_used,524449},{binary,6236480},{code,9692859},{ets,721496}]
Exporting tree took 434 milliseconds
Importing tree took 100 milliseconds
Memory footprint [{total,155941512},{processes,123734808},{processes_used,123734384},{system,32206704},{atom,540873},{atom_used,524449},{binary,10401144},{code,9692859},{ets,721496}]
Garbage collect
Memory footprint [{total,39660504},{processes,8257520},{processes_used,8256968},{system,31402984},{atom,540873},{atom_used,524449},{binary,9781760},{code,9692859},{ets,721496}]

Post change:

Generating Keys took 2416 milliseconds
Memory footprint [{total,284678120},{processes,258349528},{processes_used,257758568},{system,26328592},{atom,893161},{atom_used,878150},{binary,1013880},{code,11770188},{ets,774224}]
Generating new tree took 0 milliseconds
Loading tree took 2072 milliseconds
Memory footprint [{total,49957448},{processes,17244856},{processes_used,16653896},{system,32712592},{atom,893161},{atom_used,878216},{binary,7397496},{code,11770188},{ets,774224}]
Exporting tree took 448 milliseconds
Importing tree took 108 milliseconds
Memory footprint [{total,46504880},{processes,11197344},{processes_used,10606384},{system,35307536},{atom,893161},{atom_used,878216},{binary,9992112},{code,11770188},{ets,774224}]
Garbage collect
Memory footprint [{total,47394048},{processes,12223608},{processes_used,11632520},{system,35170440},{atom,893161},{atom_used,878216},{binary,9855008},{code,11770188},{ets,774224}]

[martinsumner]

Note the map version has a higher memory footprint. However, it should lead to less churn on the memory.

The post change benefit in memory footprint post import of tree is because of a manual garbage_collect() call, not because of any efficiency in the load process.

[ThomasArts]

Here the assertion is clear enough