Milestone 8: Ensure metadata for all possible storage variables is correct

[xermicus]

The polkadot-api allows storage variables to be read without executing any code. This makes it possible to inspect the state of a contract without running any code.

This is currently implemented for primitive types, structs and fixed-size arrays. Mappings and variable length arrays are not implemented. If at all possible, then implement mappings and variable length arrays. It may be necessary to adjust the solang implementation of these types to match the polkadot-api side, alternatively this may require some extensions on the polkadot-api side.

This feature should have tests for all storage types, including primitives, 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.