Add CSPoppy

@moritayasuaki has an implementation of it forked off of my structures repository at:

https://github.com/moritayasuaki/structures/blob/succinct/src/Data/Vector/Succinct/Poppy.hs

Start by reading the linked src/Data/Vector/Succinct/Poppy.hs implementation and then inspect the succinct repository to find where a CSPoppy structure belongs. Confirm the intended API and integration points before adding it; done means CSPoppy is incorporated into the project and its behavior is covered by the repository's existing checks.

Written by the indexing model from the issue text.