Add nullability annotations to `binaryninjacore.h`

In addition to the obvious DX improvements, this would:

• Fix type errors/inaccuracies in the python API
  • Instead of always having to type return values Optional[foo] for core APIs that return a pointer we can just type it as returning foo if they're marked as returning non-null
  • Check at runtime that None isn't passed to core APIs that don't take a nullable parameter
• Allow adding nullability annotations to the C++ API
• Probably expose a lot of bugs in existing code using the API where there are potential null derefs

Start by reviewing binaryninjacore.h and tracing how its pointer declarations feed the Python and C++ APIs. Done means the relevant core APIs have accurate nullability annotations, nullable parameters are distinguished from non-null ones, and the resulting type and runtime checks reflect those annotations.