adapt.ts: add support for core::integer::u256

[cfal]

Usage related changes

Fixes handling for core::integer::u256, which should be represented as 2 felts.
Partly addresses #341

Checklist:

• Formatted the code
• No linter errors + tried to avoid introducing linter warnings
• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Updated the test directory (with a test case consisting of network.json, hardhat.config.ts, check.ts)
• Linked issues which this PR resolves
• Created a PR to the plugin branch of starknet-hardhat-example:
  • < EXAMPLE_REPO_PR_URL >
  • Modified test.sh to use my example repo branch
  • Restored test.sh to to use the original branch (after the example repo PR has been merged)
• All tests are passing (for external contributors who don't have access to the CI/CD pipeline)

[FabijanC]

Looks good, nothing to add