feat: use map to store policy index

[imp2002]

Fix: #278

Benchmark previous:

Benchmark current:

[casbin-bot]

@tangyang9464 @seriouszyx @elfisworking please review

[codecov-commenter]

Codecov Report

Merging #283 (153f172) into master (5f78e43) will increase coverage by 0.41%.
The diff coverage is 97.36%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   66.33%   66.75%   +0.41%     
==========================================
  Files          45       45              
  Lines        2225     2262      +37     
  Branches      388      394       +6     
==========================================
+ Hits         1476     1510      +34     
+ Misses        636      633       -3     
- Partials      113      119       +6     

Impacted Files	Coverage Δ
src/main/java/org/casbin/jcasbin/model/Model.java	100.00% <ø> (ø)
src/main/java/org/casbin/jcasbin/model/Policy.java	78.43% <96.42%> (+1.18%)	⬆️
.../main/java/org/casbin/jcasbin/model/Assertion.java	73.91% <100.00%> (+0.57%)	⬆️
...c/main/java/org/casbin/jcasbin/persist/Helper.java	76.92% <100.00%> (+6.92%)	⬆️
src/main/java/org/casbin/jcasbin/util/Util.java	61.53% <0.00%> (-6.60%)	⬇️
src/main/java/org/casbin/jcasbin/rbac/Role.java	89.74% <0.00%> (-5.26%)	⬇️
...ain/java/org/casbin/jcasbin/main/CoreEnforcer.java	79.19% <0.00%> (+0.20%)	⬆️
...a/org/casbin/jcasbin/main/DistributedEnforcer.java	33.33% <0.00%> (+0.95%)	⬆️
...va/org/casbin/jcasbin/rbac/DefaultRoleManager.java	83.51% <0.00%> (+4.39%)	⬆️
...java/org/casbin/jcasbin/main/InternalEnforcer.java	50.95% <0.00%> (+5.00%)	⬆️

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[hsluoyz]

@tangyang9464

[tangyang9464]

LGTM

[github-actions]

🎉 This PR is included in version 1.25.0 🎉

The release is available on:

• GitHub release
• v1.25.0

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