__typeid

[Andrei Alexandrescu]

I implemented as far as I could from the classinfo API:

https://github.com/dlang/druntime/pull/3174

I couldn't get my hands on number of things, such as:

* the vtable (only vptr is accessible, not the size)
* the vtables of implemented interfaces
* the info whether a class is a COM class (I was surprised to learn that 
IUnknown is only defined in Windows-specific modules... it should be 
available to Posix too.
* hasOffTi