feat: Initial migration from rust 2018 to rust 2021

[SiddheshKanawade]

Steps followed to migrate from rust 2018 to 2021:

1. cargo update
2. cargo fix –-edition
3. updated the cargo.toml --edition
4. ran cargo build and cargo test to check whether changes are implemented and code is running. The warnings were resolved after the update in main repository which included the change of syntax of def_package! compatible with latest rhai edition
5. Updated the Cargo.toml file dependencies to their latest version
6. cargo +nightly udeps to check the unused dependencies, found none
7. cargo outdated to check outdated dependencies, found some, updated via cargo update
8. cargo build and cargo test to verify the changes are implemented successfully

Following is the output when ran cargo outdated

Following is the output when ran cargo +nightly udeps

[casbin-bot]

@smrpn @hackerchai @PsiACE @GopherJ please review

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: SiddheshKanawade
:x: macbook air

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[hsluoyz]

@SiddheshKanawade plz fix:

[codecov[bot]]

Codecov Report

Merging #290 (908cab6) into master (9320add) will decrease coverage by 0.00%. The diff coverage is n/a.

:exclamation: Current head 908cab6 differs from pull request most recent head 972f773. Consider uploading reports for the commit 972f773 to get more accurate results

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
- Coverage   81.03%   81.03%   -0.01%     
==========================================
  Files          23       23              
  Lines        3338     3337       -1     
==========================================
- Hits         2705     2704       -1     
  Misses        633      633              

Impacted Files	Coverage Δ
src/rbac/default_role_manager.rs	98.12% <0.00%> (-0.01%)	:arrow_down:

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[SiddheshKanawade]

@PsiACE I have already signed the CLA, but it isn't showed in the status

[PsiACE]

@PsiACE I have already signed the CLA, but it isn't showed in the status

ref https://github.com/casbin/casbin-rs/pull/290#issuecomment-1058088835

macbook air seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.

[SiddheshKanawade]

@PsiACE I am getting error while adding email address and I read the official documentation and it says I can't update the email address for previous commits, so I will be closing this PR and will make a new PR, will this work?