freeze_struct improvements / custom linting framework

[Sam Johnson]

#513 introduced the #[freeze_struct] attribute which allows you to "lock" a storage struct such that any changes to it result in a new hash code that is enforced at compile time.

A few additional features would enhance this:

• ignore changes to generic name in hash code calculation (verify that this is actually harmless)
• ignore changes to field names in hash code calculation (verify that this is actually harmless)
• custom linting framework
• an automatic lint requiring this on storage structs

[Sam Johnson]

fixed by #724