Implement new Contract Design

[nielstron]

The contract design proposed in #306 should be implement with the following additions

• opshin build recursively searches the current directory for python files containing contracts. all found contracts are built.
• the current functionality is mainted in a form similar to aiken export (Add aiken export aiken-lang/aiken#843) to be reused for i.e. Plutus Bench

Bug bounty: 1500 ADA

[irispigo]

This issue wasn't solved, yet? If so, I'll fix it.

[nielstron]

No, not fixed yet :)

[nielstron]

Closing as duplicate of #306