Add ArrayStore data structure (#395)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

The ArrayStore will be part of an upcoming archive refactor in pyribs.

## TODO

<!-- Notable points that this PR has either accomplished or will
accomplish. -->

## Questions

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## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/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

HISTORY.md

@@ -7,6 +7,7 @@
 #### API
 
 - Add GradientOperatorEmitter to support OMG-MEGA and OG-MAP-Elites ({pr}`348`)
+- Add ArrayStore data structure ({pr}`395`)
 
 #### Improvements
 

ribs/archives/__init__.py

@@ -17,6 +17,7 @@
     ribs.archives.CVTArchive
     ribs.archives.SlidingBoundariesArchive
     ribs.archives.ArchiveBase
+    ribs.archives.ArrayStore
     ribs.archives.AddStatus
     ribs.archives.Elite
     ribs.archives.EliteBatch
@@ -28,6 +29,7 @@
 from ribs.archives._archive_base import ArchiveBase
 from ribs.archives._archive_data_frame import ArchiveDataFrame
 from ribs.archives._archive_stats import ArchiveStats
+from ribs.archives._array_store import ArrayStore
 from ribs.archives._cqd_score_result import CQDScoreResult
 from ribs.archives._cvt_archive import CVTArchive
 from ribs.archives._elite import Elite, EliteBatch
@@ -39,6 +41,7 @@
     "CVTArchive",
     "SlidingBoundariesArchive",
     "ArchiveBase",
+    "ArrayStore",
     "AddStatus",
     "Elite",
     "ArchiveDataFrame",