Use compact regions for our search state.

[recursion-ninja]

GHC 8.2.1 introduces the primitive memory management helper library known as "compact regions".

If we define our search state as a compact region, we will get the benefit of:

• Almost certainly better garbage collector performance.

• An easy language defined way to serialize and reload our search states to and from disk.

[Boarders]

There is a first draft approach here: https://github.com/amnh/PCG/tree/CompactExperiment

This currently returns: pcg: compaction failed: cannot compact functions

This is because some of the decoration fields have functions like:

symbolChangeMatrixData :: Word -> Word -> Word transitionCostMatrixData :: c -> c -> (c, Word)

It may still be possible to use compact regions by re-writing how the metric data of these types are stored in the decoration.

[recursion-ninja]

@Boarders I believe that is has been implemented. Should we close the issue?