Check structs in storage layout validation

[spalladino]

When comparing storage layouts, we are only comparing by variable name and type identifier, without checking if the type changed at all. For instance, these two contracts should be incompatible:

contract V1 {
  struct MyStruct { uint256 s1; }
  MyStruct a;
  uint256 b;
}

contract V2 {
  struct MyStruct { uint256 s1; uint256 s2; }
  MyStruct a;
  uint256 b;
}

We need to add recursive checks for layout of structs.

Follows from #92.