Refactor loading

[FabijanC]

Usage related changes

None

Development related changes

• Simplify loading
• Avoid needing the re_execute_on_init flag
• Unify load_events and load_events_custom_path
• Remove loading logic from Starknet::new. Create a new method Starknet::load which uses Starknet::new and loads.

Checklist:

• [x] Checked out the contribution guidelines
• [x] Applied formatting - ./scripts/format.sh
• [x] No linter errors - ./scripts/clippy_check.sh
• [x] No unused dependencies - ./scripts/check_unused_deps.sh
• [x] No spelling errors - ./scripts/check_spelling.sh
• [x] Performed code self-review
• [x] Rebased to the latest commit of the target branch (or merged it into my branch)
  • Once you make the PR reviewable, please avoid force-pushing
• [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