improve the code robustness of slist

[wjiec]

1. use explicit panic instead of runtime errors such as null pointers
2. add test cases for SList.Front and SList.Back
3. sListIterator uniformly uses the same type of receiver(Go recommended)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
4 Code Smells

No Coverage information
0.0% Duplication

[coveralls]

Pull Request Test Coverage Report for Build 3923059014

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3890411830:	0.0%
Covered Lines:	1782
Relevant Lines:	1782

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💛 - Coveralls

[chen3feng]

Many thanks for your contributions.