Make ArrayStore use int32 indices

[btjanaka]

Description

The current ArchiveBase uses int32 indices (the assumption being that we will never have to deal with more than INT_MAX archive cells). This PR makes ArrayStore use int32 indices to be consistent with ArchiveBase.

TODO

• [x] Add tests
• [x] Ensure consistency across all ArrayStore methods

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go