contract implementation for KIP-103 interface

[0xxlegolas]

Proposed changes

Implementation of KIP-103 interface

• refactored smart contract implementation according to kip103 interface
• added snapshot function to get current values of state variables

References

KGP6
klaytn/kips#104

Types of changes

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• Bugfix
• New feature or enhancement
• Others

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• Lint and unit tests pass locally with my changes ($ make test)
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Related issues

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Further comments

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[ian0371]

@aidan-kwon Don't forget to rerun "go generate" from #1809 after this PR is merged.