Use unboxed array for the main matrix

[huffyhenry]

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[huffyhenry]

See https://stackoverflow.com/questions/32123475/profiling-builds-with-stack for instructions how to profile with stack and https://making.pusher.com/memory-profiling-in-haskell/ for an example.

[huffyhenry]

Asked a question on SO https://stackoverflow.com/questions/53692617/memory-usage-of-a-recursive-array-in-haskell

[huffyhenry]

Relevant: https://stackoverflow.com/questions/38206377/possible-to-do-recursive-definitions-with-unboxed-vectors/38207838

[huffyhenry]

The cause of the high memory footprint is "boxing": as I understand it, Haskell keeps the full computation of the array entries instead of the final value. Use unboxed arrays instead. Things to keep in mind:

• Only simple types can be held in unboxed arrays, how do we keep an Entry? Do we need two arrays here?
• We need to define the entries in the correct order - not hard, since N-W arrays can be filled row by row.

[huffyhenry]

Example use of the key STUArray type: https://stackoverflow.com/questions/8197032/starray-documentation-for-newbies-and-state-st-related-questions