Goals
* Reflect Apple nullability annotations in our bindings using C#8
* No warnings when building bindings
Non-Goals
* Update (add or fix) `[NullAllowed]` to match Apple headers (next phase)
* Make the generator or internal code fully nullable aware (`nowarn` is used)
Notes
* Apple's own annotations are not 100% accurate :(
* Where known issue exists we have _fixed_ our attributes to match reality :)
* We also do additional null-checks internally that might seems not required (better safe than sorry).
Generate trampoline and registrar tests that tests if a return type requires objc_msgSend or objc_msgSend_stret.
Now it's much easier to test new return types (a single line of code), which
avoids a _lot_ of copy-pasting, and makes sure all the different variations
are tested properly.
These new tests found several bugs, which are fixed in subsequent commits.