Move warn

[FabijanC]

Usage related changes

• Close Execution slower with 0.4.3 than with 0.4.2, regardless of VM #380
• The problem was that the pedersen cpp monkeypatch stopped being "atomic" because util.py imports things meant to be overriden by the monkeypatch.

Development related changes

• In __init__.py, execute from .util import warn only before warn is called
  • Helps for both Python VM and Rust VM, even though import+warn is executed only with Rust VM

Checklist:

• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/lint.sh
• Performed code self-review
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Linked the issues which this PR resolves
• Updated the tests
• All tests are passing

[mikiw]

<3