Make DisplayableException more serialization-friendly

[minestarks]

Fixes https://dev.azure.com/ms-quantum/Quantum%20Program/_workitems/edit/44472

Serializing DisplayableException to JSON causes an OutOfMemoryException to be thrown.

JSON tracing shows hundreds of objects referenced through StackFrame attempted to be serialized:

Newtonsoft.Json Information: 0 : Finished serializing System.RuntimeType. Path 'StackTrace[0].Callable.__Body__.Method.Module.Assembly.DefinedTypes[263]'.
Newtonsoft.Json Information: 0 : Started serializing System.RuntimeType. Path 'StackTrace[0].Callable.__Body__.Method.Module.Assembly.DefinedTypes[263]'.
Newtonsoft.Json Information: 0 : Finished serializing System.RuntimeType. Path 'StackTrace[0].Callable.__Body__.Method.Module.Assembly.DefinedTypes[264]'.
Newtonsoft.Json Information: 0 : Started serializing System.RuntimeType. Path 'StackTrace[0].Callable.__Body__.Method.Module.Assembly.DefinedTypes[264]'.

On principle I don't think an object designed to be serialized/displayed should have references to complicated types such as StackFrame. So I'm introducing a DisplayableStackFrame, a simple struct.

Additionally I'm cleaning up a few areas that I thought looked odd - where we were serializing the same string slightly differently, unnecessary null handling, etc.

Considered, and rejected:

• Adding a custom JSON converter for DisplayableException: designing the object to be simple in the first place seems like a more sensible approach
• Avoid serializing to JSON in the first place: This turned out to be way trickier than it sounds. Serializing results to JSON seems to be pretty baked into the IQ# engine, and the JSON results are used by the Python wrapper (namely in capture_diagnostics)