Add support for loading the contract module under test

[lassemoldrup]

Purpose

Closes #383. Depends on Concordium/concordium-smart-contract-tools#167.

Changes

Reads the environment variable set by cargo-concordium though the convenience function module_load_output. This also means that the user needs to update cargo-concordium.

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.