freeze_struct improvements

[sam0x17]

#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)
• an automatic lint requiring this on storage structs