Better tooling around `header-translator` data files

[madsmtm]

In #329 I authored a pretty large file, which naturally lead me to thinking about how that process could be improved upon:

• Some rustdoc fork that allows one to go through, click a button, and have the source-code updated to mark it as safe (perhaps with a comment).
• Are property accessors always safe? I know that setters are not, since they may need to maintain an invariant, but it seems like if you have a reference to a type, you should always be able to access any data it holds? Maybe just for certain types (like, we could do NSData: Shared { ... }, and then all NSData properties would become safe).
• A way to make icrate generate #[method(myMethod)] pub(crate) fn myMethod_unchecked(...) for instances where one would like to modify the method but keep the name the same (e.g. NSData::bytes)
• Checks in header-translator that verify that the data entry is actually used (and e.g. not declared class X { ... } on a protocol).