using IndexSet to store policy in assertion

[GopherJ]

Previously we are using sort_unstable + dedup to avoid adding a policy the second time, but the sort operation is expensive, so I made this PR which stores policies in HashSet, in rust there is no problem to compare two arrays.

That'll probably make it more efficient.

[GopherJ]

The disadvantage is that HashSet doesn't guarantee the order of elements, so when we run get_policy it will probably gives differents results:

1st run:

[["alice", "/user/profile", "read"], ["bob", "/user/profile", "write"]]

2nd run:

[["bob", "/user/profile", "write"], ["alice", "/user/profile", "read"]]

But from an user's prospective, I think it's ok to have different orders.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c0fbedb). Click here to learn what that means.
The diff coverage is 87.87%.

@@            Coverage Diff            @@
##             master      #47   +/-   ##
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  Coverage          ?   85.89%           
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  Files             ?       12           
  Lines             ?     1992           
  Branches          ?        0           
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  Hits              ?     1711           
  Misses            ?      281           
  Partials          ?        0

Impacted Files	Coverage Δ
src/enforcer.rs	85.34% <100%> (ø)
src/adapter/file_adapter.rs	45.65% <50%> (ø)
src/internal_api.rs	84% <80%> (ø)
src/management_api.rs	88.44% <80.76%> (ø)
src/rbac_api.rs	83.24% <90.21%> (ø)
src/rbac/default_role_manager.rs	93.07% <91.89%> (ø)

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[GopherJ]

@hsluoyz How do you think?

[hsluoyz]

@GopherJ will Golang Casbin keep the order?

[GopherJ]

@hsluoyz Yes it will keep the result's order. So I'm not sure how do we handle this in rust, sort_unstable + dedup can solve the problem but it's really expensive operation: nlogn + n probably.

[hsluoyz]

@GopherJ can we have the same behavior as Golang Casbin?

[GopherJ]

@hsluoyz Yes, I just changed HashSet to IndexSet which guarantee the order.

[hsluoyz]

lgtm