Fix predeployed ERC20 storage

Usage related changes

For storage values, use short cairo string parsing instead of selector or storage var address (those should be used for keys, not values)

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[x] No linter errors - ./scripts/clippy_check.sh
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[x] Performed code self-review
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[x] Updated the docs if needed - ./website/README.md
[x] Linked the issues resolvable by this PR - linking info
[x] Updated the tests if needed; all passing - execution info