Support rename/retype of variables within struct

[ericglau]

@custom:oz-retyped-from and @custom:oz-renamed-from inside a struct are not recognized because Solidity does not expose the NatSpec of struct members.

A workaround is to specify @custom:oz-retyped-from <V1 contract name>.<V1 struct name> on the variable that uses the struct in the V2 contract. See this forum post for an example.

However, retype of a struct causes storage layout comparison to be skipped inside the struct itself, so users should manually ensure that they are compatible.

When Solidity exposes NatSpec of struct members, or if we can check the NatSpec in a different way, then:

• Include the inner type of the struct for layout comparison
• Support rename/retype annotations for variables within the struct

Note: Also consider structs used in mappings e.g. mapping(uint256 => S) store_mapping;

Related: #459