Automatic `header-translator` test generation

[madsmtm]

We should generate tests to ensure that the output of header translation is correct.

These does not necessarily need to be included within icrate, to reduce crates.io download size.

Ideas for tests:

• ClassType::class works.
• ClassType::alloc works.
• Test method encodings (dynamic vs. declared). EDIT: Done in 0bdb036.
• Test type encodings (using Entity::get_objc_type_encoding).
• Test statics. EDIT: Done in 9c07a2f
• Struct sizes and alignment.
• ...

[madsmtm]

It would be useful to avoid including dynamic method encoding debug assertions in message sends, as these often turn out to be slightly different in reality.

Examples include #566 and #526.