Support debugging in cargo-concordium.

[abizjak]

Purpose

Support the new debugging support in both concordium-std and the testing library.

The cargo concordium run command will now output any collected debug information if so desired.

cargo concordium test additionally supports --allow-debug flag, and if this is set it will enable debugging when building contracts, and also output debug output after running the tests.

Depends on

• Support for debug output concordium-base#488
• Update base to get access to debugging support in the execution engine. concordium-rust-sdk#147

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[limemloh]

Looks good.
Don't forget the changelog