[ffigen] Ease using function pointers and function tables

[dcharkes]

Some projects have function tables exposed in header files. Using such function pointers requires the user to write an asFunction with the correct Dart signature. (I believe the main use case in C code is dynamic dispatch.)

We should explore if we can generate the Dart code that does the asFunction.

Some use cases:

• 'Pointer<NativeFunction<T Function()>> ptr; T Function() func = ptr.asFunction(); ' When T must be 'Pointer' to pass compilation checking. sdk#54467 (comment)
• win32 COM apis (even though package:win32 has it's own code generator and is not using FFIgen)
• (I've seen more instances, but I can't remember where.)