Milestone 9: Add tooling for calls between ink! and Solidity

[xermicus]

When a contract in either ink! or Solidity is compiled, then a metadata file is written which contains all the information needed to call a contract or deploy one. Now we need to read this format and generate the code to do those calls, so that Solidity can call ink! and vice versa.

This feature should have tests at

https://github.com/hyperledger-labs/solang/tree/main/integration/substrate

Success means that a pull request has had all its review comments addressed and it is merged.