fix: clear stale callback after execution

[0xjc65eth]

Summary

• Clear callbackAddress after a successful _executeActions run.
• Prevent a flow that arms SETCALLBACK but never receives/processes the callback from leaving a stale callback address in executor storage.
• Add a regression test that executes SETCALLBACK alone and asserts storage slot 1 is cleared afterward.
• Run rustfmt; this also formats pre-existing long include_bytes! lines in src/lib.rs.

Fixes #8.

Tests

• git diff --check
• cargo +stable fmt
• cargo +stable test test_execute_actions_clears_unprocessed_callback (blocked locally: build.rs could not parse solc output, likely because the local Solidity compiler invocation produced no JSON output)