Improve support for debuggers

[madsmtm]

It would be especially cool if we could re-use the debugging support that LLDB already contains for Foundation types like NSString and NSArray.

The unstable #![debugger_visualizer] attribute is probably the way to go, once it supports LLDB. Support for other debuggers is also desired, but probably less important (since the expectation is that users are going to be developing on macOS).

LLDB has a way to change how types are debugged, so I might try writing a python script that users can command script import (similar to how Rust's LLDB support currently work, though I think we might need to improve that too, see also vadimcn/codelldb#997).

[madsmtm]

The #![debugger_visualizer(...)] attribute is now stabilized, though without support for LLDB - we/I will probably have to do the work ourselves if we want that support.

[madsmtm]

It would also be nice if we could somehow make instance variables show up by default if inspecting Objective-C code in Xcode (even if they're not exactly pretty).

Currently they're just hidden (exactly like for imported Swift classes), which is quite confusing!

See also #459.