Cross-contract unit test PoV

[h4x3rotab]

A work-in-progress PoV of Green's idea: Supercolony-net/openbrush-contracts#136 (comment)

• Enable the entry points (deploy & call) in unit tests
• Contract registry to track entrypoints, code hashes, and the account id
• Call stack management in the offchain env
• Pass the examples/delegator unit test
• Error & revert handling
• Compatible with on-chain api (minor change)

Emulate unimplemented offchain apis:

• decode_input
• return_value (changed Never return type to ())
• set_code_hash
• invoke_contract (changed to two-stage commit: _begin & _end(), without error handling)
• invoke_contract_delegate
• instantiate_contract (changed to two-stage commit: _begin & _end(), without error handling)
• is_contract
• caller_is_origin
• code_hash
• own_code_hash

[h4x3rotab]

Closing in favor of Supercolony's more mature work: Supercolony-net#4