Improve type testing

[alfonsogarciacaro]

Following the increased reflection support we could try to overcome the current limitations in Fable's type testing:

• Allow testing interfaces (except the NoMangle ones as they're supposed to represent JS objects). For that we should include the implemented interface names in the type (we already did something similar in Fable 1).
• Allow testing generic types. For that we need to inject the name of generic arguments to the constructors.

[Shmew]

This would be great look at this gross code 😅

[alfonsogarciacaro]

I've tried different approaches: injecting generics as arguments, wrapping the constructors, trying to infer the generics from private fields... but I was not convinced by anyone, as none was perfect and they introduced complications without opening interesting scenarios. The only cases I've seen this is need is to cross-compile code for .net and Fable, so for now I will just enable testing generic types with a warning saying the generics are not taken into account. Hopefully that should be enough for most cases.