Type field object not initialized properly for inherited objects

[dandevelo]

More details here:

https://forum.nim-lang.org/t/3875

[jangko]

there is no way for the compiler to auto initialize ptr object if it not allocated first.

here is my proposal(in no particular order or preference):

1. accept @GULPF answer as standard practice, and put it into doc/manual
2. fix system.create/system.createShared to initialize ptr object and put it into doc/manual
3. create new/newShared similar with C++ resulting in initialized ptr object and put it into doc/manual
4. if this issue considered as a bug, we can fix ptr object contruction behave like ref object construction except it need to be manually freed/deallocated.
5. point number 1,2,3,4 can coexist together without conflict.