Milestone 6. Ensure all return types have correct metadata

[xermicus]

Currently if the return type of a function is an address, it is not rendered as an account in polkadot.js UI. Ensure that all types have the correct type so they will be rendered correctly.

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.